Finding Tumblr’s Favorite Animal!

Round 2 - Arthropoda - Cephalocarida

(Sources - 1, 2, 3)

Cephalocarida is a class of crustaceans commonly referred to as “Horseshoe Shrimp.” There is only one known family, Hutchinsoniellidae, with 12 living species.

Both the larvae and adults are found on the sea floor, feeding on marine detritus. They are small, with an elongated, pigmentless body 2 to 4 mm in length. They have a large head, the hind edge covering the first thoracic segment. They have two pairs of antennae, with the second pair located behind the mouth. The mouth is covered by a large upper lip and flanked by a pair of mandibles. The first pair of maxillae are very small, but the second pair is similar to a pair of legs, a possible holdover from primitive arthropod ancestors. Neither the adults nor the nauplius larvae have eyes. The thorax consists of 9 limb-bearing segments, followed by 10 limbless abdominal segments and a telson. Cephalocaridans are hermaphroditic, and carry their eggs with them. Mating has yet to be observed.

Fossils of cephalocaridans have been dated to the Late Cambrian.

Round 2 - Arthropoda - Branchiopoda

(Sources - 1, 2, 3, 4)

Branchiopoda is a class of small, mainly freshwater crustaceans that feed on plankton and detritus. They are comprised of 9 orders: Anostraca (“Fairy Shrimp”), Anomopoda (“Water Fleas”), Ctenopoda (also “Water Fleas”), Cyclestherida (“Clam Shrimp”), Laevicaudata (also “Clam Shrimp”), Spinicaudata (also “Clam Shrimp”), Haplopoda (“Predatory Water Fleas”), Onychopoda (“Water Fleas” again), and Notostraca (“Tadpole/Shield Shrimp”).

Branchiopods are found mainly in freshwater, including temporary pools and hypersaline lakes, and some in brackish water. Those that live in temporary pools are known for having eggs that can dry out for long periods of time and hatch once they are submerged in water, as an adaptation to drought. Only two families, one in Onychopoda and one in Ctenopoda, contain marine species. Most eat detritus or plankton, catching them in the setae on their appendages. Notostracans are opportunistic omnivores that will feed on algae, bacteria, other branchiopods like Anostracans, and even small fish.

Branchiopods are characterized by the presence of gills on many of the animals’ appendages, including the mouthparts. Most have compound eyes and a carapace. In the Clam Shrimp, the carapace prevents the use of the legs for swimming, so the antennae are used for locomotion instead, as they are in nauplius larvae.

The oldest known branchiopod was Rehbachiella kinnekullensis of the Upper Cambrian. Notostracans in particular have a good fossil record, with the oldest known species being Strudops goldenbergi from the Late Devonian. Notostracans are often described as “Living Fossils” due to their lack of major morphological change over 250 million years.

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Round 2 - Arthropoda - Tantulocarida

(Sources - 1, 2, 3)

Tantulocarida is a class of tiny crustaceans which live as ectoparasites of copepods, isopods, tanaids, amphipods, and ostracods. They contain about 33 species within 5 families.

Tantulocarida are not only the smallest crustaceans, but also the smallest arthropods: less than 0.3 millimetres in length, the smallest (Tantulacus dieteri) being less than 85 micrometres long. They have an unsegmented, sac-like thorax and a reduced abdomen. They have no eyes.

The larvae consist of a head with an “oral disc” and no appendages, a 6-segmented thorax with 6 pairs of legs, and a limbless abdomen with 1-6 segments and a telson. Their life cycle is unique in that they transform directly from a non-feeding, free-swimming larvae into an adult parasite without any moults in between. Once the larvae finds a host, it will pierce its host’s cuticle with a stylet, upon which a rootlet system grows through the hole and into the host’s tissue. The larvae will degenerate, losing all muscle mass and becoming a permanent part of the host. The adult form develops inside the larva, and can become either a free-living, non-feeding, sexually-producing male or female, or a sac-like parthenogenetic female which will release fully developed tantulus larvae (image 2). The sexually-producing females may release nauplius larvae instead. Not much else is known about these bizarre crustaceans!

Round 2 - Arthropoda - Malacostraca

(Sources - 1, 2, 3, 4)

Malacostraca is the second largest class of crustaceans, and what most people picture when they hear the word crustacean! It contains over 40,000 species separated into 17 orders: Leptostraca, Stomatopoda (“Mantis Shrimp”), Decapoda (“Crabs”, “Lobsters”, “Crayfish”, “Shrimp”, and “Prawns”), Euphausiacea (“Krill”), Thermosbaenacea, Mysida (“Opossum Shrimp”), Stygiomysida, Lophogastrida, Spelaeogriphacea, Mictacea, Bochusacea, Cumacea (“Hooded/Comma Shrimp”), Tanaidacea, Amphipoda, Isopoda, Anaspidacea, and Bathynellacea. Many are scavengers, some are predators, some are herbivores, some are filter feeders, and some are parasites.

Malacostracans live worldwide, in marine, freshwater, and terrestrial environments, and have a large diversity of body forms. They are united by their segmentation of 20-21 body segments divided into a 5-segmented head, an 8-segmented thorax, and a 6-segmented abdomen with a telson, except in Leptostraca which has 7 abdominal segments. They have a pair of jointed appendages on each abdominal segment, though some groups have lost them secondarily. In some, three thoracic segments may be fused with the head to form a cephalothorax, the associated legs becoming maxillipeds. They have two pairs of antennae, which often branch into two parts. Their mouthparts have a pair of mandibles, maxillules, and maxillae. Many taxa have compound eyes on moveable stalks. Some have a carapace which covers the head, part or all of the thorax and some of the abdomen. The carapace may be fused with some of the thoraacic segments or hinged with two parts. This is one of the most diverse classes in the animal kingdom, and their anatomy and behavior would be hard to summarize further in just one paragraph!

The oldest malacostracans are the Leptostracans, which first appeared as fossils from the Cambrian period.

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Round 2 - Arthropoda - Thecostraca

(Sources - 1, 2, 3, 4)

Thecostraca is is a class of crustaceans, many of which have planktonic larvae which become sessile or parasitic as adults. The most well-known group are the Barnacles (subclass Cirripedia), but Thecostraca also includes the parasitic Ascothoracida, and the mysterious Facetotecta.

Facetotecta, comprising only the genus Hansenocaris, are known only from their larvae (image 3) and adults have yet to be recognized, though some scientists believe they may actually be larval tantulocaridans.

Ascothoracidans are parasites of echinoderms and cnidarians. Most genera are meso and endoparasitic (living inside the host) while some are ectoparasitic (living on the outside of the host). They are similar in anatomy to copepods, with six pairs of legs, an abdomen with four segments, a telson, and a bivalved carapace. They feed on their host via piercing and sucking mouthparts, and some more advanced species also absorb nutrients through the carapace. They are sexually dimorphic, in many cases so much so that the smaller males will live inside the larger female’s mantle cavity.

Barnacles (subclass Cirripedia) are more well-known than other Thecostracans. Adult barnacles are sessile filter feeders, except for the infraclass Rhizocephala, which are parasites of other crustaceans. Barnacles attach themselves to a surface as adults, be that a rock, the shell of a mollusc, a ship, or a large animal such as a whale. They come in two common forms: acorn barnacles which grow their shells directly on a surface (image 4) and goose barnacles which attach themselves via a stalk (image 1). Barnacles have a carapace made of six calcareous plates, with a lid made of four more plates. They attach themselves to the substrate by means of a cement gland at the base of their antennae. Eight pairs of thoracic limbs, called cirri, extend from the carapace to filter plankton from the water and bring it towards the mouth. The hairs on these limbs are very sensitive to touch, and help the barnacle sense the world around them. They also have three simple eyes (ocelli) which can sense changes in light, allowing them to close their plates quickly if a shadow is detected.

Thecostracans have nauplius larvae, characterised by a head with antennules, antennae, mandables, and a single eye, three pairs of limbs, a carapace, and a telson. Barnacle larvae are brooded by the parent until their first moult, after which they are released to swim freely using setae.

The oldest known thecostracan fossil is dated from the Middle Cambrian. Traces of the parasitic forms have been dated from the Cretaceous.

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Round 2 - Arthropoda - Copepoda

(Sources - 1, 2, 3, 4)

Copepoda is a class of small crustaceans found in nearly every freshwater and saltwater biome, including the arctic. Some are planktonic, some live in sediment (benthic), some live underground in sinkholes or caves, some are parasitic, and some even live in wet terrestrial places such as bogs and the water-filled cups of bromeliads. They are small, usually 1 to 2 mm long, with a teardrop-shaped body and two pairs of antennae. Some polar copepods can reach up to 1 cm long. Most copepods have a single compound eye, usually bright red and in the centre of their transparent head. Subterranean species may be eyeless, and a couple genera have two eyes. Free-living copepods have a head fused with the first one or two thoracic segments, with the remainder of the thorax being comprised of three to five limbed segments. The first pair of appendages are maxillipeds, limbs used for feeding. The second pair beat like oars, aiding in swimming. They have a narrow abdomen with five leg-less segments, with tail-like rami at the tip. Meanwhile, the anatomy of parasitic copepods are so widely diverse that I simply do not have space to talk about it here. Copepods have incredibly fast reflexes, due to well-developed myelin sheaths, allowing them to escape predators at high speeds, often porpoising out of the water. Like ostracods, many species also use bioluminescence as a defense mechanism, using it to distract predators (see gif below).

When they are ready to mate, some copepod females leave a trail of pheromones for males to follow. When mating, the male will grip the female with his antennae and produce an adhesive spermatophore, then transfer it to the female’s genital opening. After fertilization, the eggs will sometimes be laid directly into the water column, or, in some species, the female will carry them in a sac until they hatch. In some pond-dwelling species, the eggs can remain dormant in the case of the pond drying up, waiting to hatch until more favorable conditions are present. The larvae hatch with a head and a tail but no true thorax or abdomen. In fact, the larvae look so different from their adult forms that many of them were once thought to be different species! They will moult 5-6 times before becoming a copepodid larva which resembles the adult, sans some limbs and segments. After 5 more moults they will reach adulthood.

The oldest known fossils of copepods are from the Late Carboniferous, but due to their small size and fragility, they are rare in the fossil record. However, these fossilized copepods seemed to belong to an extant (still living) family, meaning that copepods may have already reached the forms they are in now by the Carboniferous.

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Round 2 - Arthropoda - Ostracoda

(Sources - 1, 2, 3, 4)

Ostracoda are a class of crustaceans commonly known as “Seed Shrimp,” grouped into 7 orders. Ostracods are unique among crustaceans for their carapace which consists of two valves, superficially resembling the shell of a tiny clam.

Ostracods live in both freshwater and marine environments, and some even live in humid soils. Most live either as plankton or in sediment. The ostracod body is not clearly divided into segments, consisting of a head and thorax region. Their head has two pairs of well-developed antennae that are used to swim through water, a pair of mandibles, and a pair of maxillae. The thorax has three main appendages. The first pair’s function varies between species. It can be used for feeding, for walking, or for mating. The second pair is mainly used for locomotion, and the third is used for walking or cleaning, and it is reduced or absent in some species. Ostracods sense the world around them via sensitive hairs on their bodies and appendages, though the order Myodocopina also has compound eyes, and the Podocopids have simple eyes as larvae. They have a wide variety of diets and can be carnivores, herbivores, or scavengers, though most are filter feeders.

Male ostracods have two penises, corresponding to two genital openings (gonopores) on the female. Mating often occurs in a swarm, with large groups of females joining large groups of males. Some species are wholly or partially parthenogenetic, consisting almost entirely of females. Some species care for their young, releasing them after their first moult, some release their eggs directly into the water, while others glue their eggs to vegetation or a firm surface. Glued eggs often do not hatch until conditions are favorable. Larvae add on more legs with each moult until achieving their adult form.

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Round 2 - Arthropoda - Mystacocarida

(Sources - 1, 2)

The class Mystacocarida comprises one family: Derocheilocarididae, commonly called “Mustache Shrimp.”

Mystacocarids are tiny pigmentless crustaceans, less than 1 mm (0.04 in) long, that live in the spaces between sand grains on intertidal beaches. They have been found along the coasts of South and North America, southern Africa, and the western Mediterranean, though they likely exist elsewhere and simply haven’t been sampled for. They have a cylindrical body with ten segments: five thoracic and five abdominal. On their head they have two pairs of long antennae, a pair of limb-like mandibles and two pairs of maxillae. Fine hairs allow the animal to strain detritus from the water to feed on. They have five pairs of limbs. The first pair has been modified into maxillipeds, used for feeding. The other four pairs of limbs are small, unsegmented lobes used for swimming, with the last pair being modified in males. Their abdomen ends in a supra-anal plate, a telson, and a pair of large pincer-like furca. Their tiny larvae hatch with three head segments which are covered by a single carapace. They later moult into their adult form.

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Round 2 - Arthropoda - Ichthyostraca

(Sources - 1, 2, 3, 4)

Our first class of crustaceans is the parasitic Icthyostraca. This class comprises the subclasses Branchiura (“Fish Lice” or “Carp Lice”) and Pentastomida (“Tongue Worms”).

Branchiurans (images 1-3) have a flattened, oval body covered by a carapace, four segments each with a pair of swimming legs, and a pair of compound eyes. They can be a few millimetres to over 30 mm (1.2 in) long, with females usually somewhat larger than the males. Adults have toothed hooks on their mandibles that allow them to cling to their hosts, and some species also have a pair of suction cups. Their mouthparts and the first pair of antennae are modified to form a hooked, spiny proboscis. Branchiurans feed primarily on marine and freshwater fish, but can also be found on other aquatic organisms such as invertebrates, salamanders, tadpoles and alligators. Some species feed on the blood of their host, while others feed on mucus and dead scales, not harming the fish. However, when they reach high enough densities, they can kill fish. Branchiurans are not permanently attached to their hosts, and females may leave them for up to three weeks to lay eggs on the surfaces of plants or rocks. When the larvae (image 2) hatch, they will swim off to select their own host.

Meanwhile, Pentastomids (image 4) parasitize the respiratory tracts of both aquatic and land vertebrates. Adults can reach 1 to 14 cm (0.4 to 5.5 in) in length, with females being larger than males. They have a mouth and two pairs of hooked legs which are used to attach to their host. They feed entirely on blood, except for the genus Linguatula which lives in the nasal cavity of carnivorous mammals and feeds mainly on mucus and dead cells. Pentastomids lay their eggs within their host, which are then either coughed out by the host or leave the body through the digestive system. At least one species has a free-living larvae, but most depend on the eggs being eaten by an intermediate host, which is usually either a fish or a small mammal. The larva hatches within the intermediate host, forming a cyst within the host’s body. It is initially round with four or six short legs, and will moult several times until it reaches its adult form. The pentastomid arrives in its main host when the intermediate host is eaten by the main host, crawling into the respiratory tract from the esophagus.

Exceptionally preserved fossils of pentastomids date back to the Cambrian. They seem to be relatively unchanged, and may be a clue as to the link between arthropods and other invertebrates.

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Round 2 - Arthropoda - Symphyla

(Sources - 1, 2, 3, 4)

Symphyla is a class of soil-dwelling arthropods commonly called “Garden Centipedes” or “Pseudocentipedes”, though they are not that closely related to centipedes. They are broken into two families: Scutigerellidae and Scolopendrellidae.

Symphylans live deep in soil, under stones, and in decaying wood, feeding on decaying vegetation, but can also feed on the seeds, roots, and root hairs of plants, and at least one species is predatory. They are very small, generally 2 to 10 millimetres long, and lack pigment. They have no eyes, using a pair of long, segmented antennae to perceive the world around them. Like centipedes, they also have Organs of Tömösváry at the base of their antennae, which may be used to sense vibrations. While most arthropods breathe through spiracles on the sides of their body, symphylans have a pair of spiracles on either side of their head. They have 14 segments, with legs on the first 12, though the first pair is absent or vestigial in some species. The 13th segment is fused to the 12th, and bears a pair of spinnerets. The 14th segment has a pair of long sensory hairs (trichobothria), the anus, and a small telson. Symphylans use their spinnerets to create long threads of silk. They use this mainly in reproduction, but may also spray silk as a defense or even use it to suspend themselves in the air.

Like centipedes and pauropods, symphylans do not copulate. Males deposit spermatophores on small stalks of webbing and leave them for the female. When she finds them, the female picks them up in her mouth and lays her eggs. She attaches her eggs to the sides of crevices or moss, smearing the sperm over them as she does so. The young will hatch with 6-7 pairs of legs, adding additional segments and legs with each moult. They can live up to 4 years, moulting throughout their life, though they do not add on more segments or legs after they reach their adult form.

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Round 2 - Arthropoda - Pauropoda

(Sources - 1, 2, 3, 4)

Pauropoda is a class of small, pale, millipede-like arthropods. They are composed of two orders: the Hexamerocerata with eleven pairs of legs and telescopic antennae and the Tetramerocerata with eight to ten pairs of legs and short, non-telescopic antennae.

Pauropods live in soil and leaf litter worldwide, feeding on mold, fungi, and occasionally the root hairs of plants. They are small, soft-bodied animals, only 0.3 to 2 mm in length. They do not have eyes, though they do have sensory organs which detect light. Long, sensory hairs called trichobothria are located throughout their body segments. Their anal plate is distinctive, helping distinguish between species.

Like centipedes, pauropods do not mate directly. Males place a sperm packet on the ground for females to find. Females deposit the fertilized eggs on the ground and subsequently abandon them. Tetrameroceratans hatch with three pairs of legs and Hexameroceratans with six, adding a new segment after each molt in a process called anamorphosis. Pauropods will continue molting even after their adult number of segments is reached, without adding any further segments or legs.

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Round 2 - Arthropoda - Diplopoda

(Sources - 1, 2, 3, 4)

Animals in the class Diplopoda are commonly called “Millipedes.” They comprise the orders Glomeridesmida (“Slug Millipedes”), Glomerida (“Northern Pill Millipedes”), Sphaerotheriida (“Giant Pill Millipedes”), Platydesmida (“Flat Millipedes”), Polyzoniida (“Camphor Millipedes”), Siphonocryptida, Siphonophorida, Julida, Spirobolida, Spirostreptida, Callipodida, Chordeumatida (“Sausage Millipedes”), Stemmiulida, Siphoniulida, Polydesmida, and Polyxenida (“Bristly Millipedes”).

Millipedes are detritivores, eating decaying leaves and other dead plant matter, making them important for cycling soil. Some species eat fungi or drink plant sap. Most species protect themselves with various chemical secretions, but they will also coil into a ball to protect their legs and vital areas when threatened. Polyxenids lack poison and a hard exoskeleton, so their defense involves barbed hairs that detach and stick to the mouths of predators.

Millipede heads are generally round and flattened, consisting of a pair of mandibles, a plate-like “jaw lip” called a gnathochilarium, a pair of small sensory antennae, and simple compound eyes. Some species have secondarily lost these eyes. Many species also have a pair of sensory organs called the Tömösváry organs at the bases of their antennae, which may measure humidity and/or light levels. Millipede bodies may be flattened or cylindrical, and they can be anywhere from 2 mm (1⁄16 in) to 35 cm (14 in) in length, and have from 11 to over 300 body segments. The segment behind their head is called a collum and is legless. The second, third, and fourth body segments are called haplosegments, and each have a single pair of legs on their underside. The remaining segments are called diplosegments and have two pairs of legs each. In some millipedes, the last few segments may be legless. The final segment is called the telson. It is always legless, and contains the anus. Some millipedes (those in the superorder Nematophora) have spinnerets as well, creating silk chambers in which to molt or lay their eggs. Though their name means “thousand feet”, only one species of centipede, Eumillipes persephone, has over 1,000 feet.

Millipede reproductive behavior is diverse. Bristle millipedes reproduce similarly to centipedes: depositing spermatophores onto webs they secrete, which are later picked up by females. Other millipedes have direct reproduction, meeting up to mate. Courtship may involve tapping antennae, the male running along the back of the female, offering gifts of edible glandular secretions, or even “chirping”. Males have one or two pairs of modified legs called gonopods which are used to transfer sperm to the female during copulation. A few species are parthenogenetic, having few, if any, males. Most species simply deposit their eggs on the ground, but some construct nests of dried feces or silk. In most species, the female abandons the eggs after they are laid, but some species do provide parental care for their eggs and young. Young millipedes usually hatch after a few weeks, bearing typically only three pairs of legs. Their development is anamorphic: adding on segments and legs with each moult. Some species can live up to 10 years.

Millipedes first appeared in the Silurian period. The most famous extinct millipedes, the genus Arthropleura of the Carboniferous, grew up to 2.5 metres (8 ft 2 in) long, making them some of the largest arthropods ever known.

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Round 2 - Arthropoda - Chilopoda

(Sources - 1, 2, 3, 4)

Chilopoda is a class of arthropods commonly called “Centipedes”. They include the Scutigeromorphs (“House Centipedes”), Lithobiomorphs (“Stone Centipedes”), Scolopendromorphs (“Tropical Centipedes”), Geophilomorphs (“Soil Centipedes”), and the Craterostigmomorphs which consists of only two living species.

Centipede means “hundred feet”, though no species of centipede actually has exactly 100 feet, as their pairs of legs are always an odd number, ranging from 15 to 191 pairs of legs! All centipedes are venomous predators, delivering venom through a pair of fang-like front legs called forcipules. Prey include other invertebrates like earthworms, fly larvae, collembolans, and other centipedes. The giant Scolopendromorphs are able to tackle larger prey including vertebrates such as lizards, frogs, birds, mice, snakes, and even bats. They live in tropical or desert habitats worldwide, but only in moist environments, meaning they must be more active under cover or at night. Many species lack eyes, though some have simple eyes that allow them to distinguish light from dark. Some species use their first pair of legs similarly to antennae, sensing vibrations and even using them to “hear.” Their bodies consist of 15 or more segments, with one pair of legs attached to the sides of each segment. Their back legs, called “ultimate legs”, are modified depending on species, and can be elongated and thin, thickened, or pincer-like. They are not used for walking, but instead can be used in defensive displays, for hunting, or for interspecies fighting, and they often play a role in mating rituals and are usually sexually dimorphic. Centipedes range from a few millimetres to 30 cm (1 foot) long.

Centipedes are generally solitary, not even meeting up to mate. Males will leave spermatophores on the ground for females to pick up. However, females provide parental care, coiling around both their eggs and young and grooming them (first image). Some species grow anamorphically: adding on segments with each moult until they reach their adult number of segments. Some species grow epimorphically: growing all their segments and legs as an embryo before hatching, and not adding on any more when they moult.

Centipedes arose during the Late Silurian, but were considerably rare until the Triassic Period. The modern Scolopendromorphs and Scutigeromorphs appeared in the Cretaceous.

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Round 2 - Arthropoda - Arachnida

(Sources - 1, 2, 3, 4)

Arachnida is a highly diverse class, ranging from the Demodex mites living on you (yes, you) right now, to the Triassic-surviving horseshoe crabs (yes, a 2019 genetics-based analysis places them within Arachnida, though this is still debated), to your friendly pest-controlling house spider. They live worldwide, mainly on land, but some also in freshwater and marine environments. There are over 110,000 named species, with more being discovered every year. They comprise the Ricinulei (“Hooded Tickspiders”), Xiphosura (“Horseshoe Crabs”), Opiliones (“Harvestmen”), Solifugae (“Camel Spiders”), Acariformes (“Acariform Mites”), Parasitiformes (“Parasitiform Mites and Ticks”), Palpigradi (“Microwhip Scorpions”), Pseudoscorpiones (“Book Scorpions”), Scorpiones (“Scorpions”), Amblypygi (“Tailless Whip Scorpions”), Schizomida (“Short-tailed Whip Scorpions”), Uropygi (“Vinegaroons”), and Araneae (“Spiders”).

Adult arachnids have 8 legs attached to a head/thorax segment called the cephalothorax. Their abdomen contains their heart and respiratory organs. Arachnids also have two other pairs of appendages that vary in function between orders. The chelicerae are mainly used in feeding and defense. In spiders, they are used to deliver venom. The second pair, called pedipalps, are used for feeding, locomotion, defense, and/or reproduction. Scorpions, Pseudoscorpions, Ricinuleids, and Horseshoe Crabs have pedipalps that end in a pair of pinchers used for defense and manipulating food. Pseudoscorpions also have a venom gland in their pinchers which they use to immobilize prey. Vinegaroons, Schizomids, Amblypygids, and most Harvestmen have raptorial pedipalps that are used for capturing prey. Solifugids use their palps as another set of legs, and spiders use theirs like hands, while the males also use theirs for mating. Mites and ticks use their palps to sense their surroundings. Mites are more diverse than other arachnids, some only having 6 or even 4 legs. Adult females of Podapolipidae only have a single pair of legs. Some arachnids have a tail-like structure called a telson. Horseshoe Crabs use their telson to flip themselves over if they are turned on their backs. Scorpions use their telson as a stinger to deliver venom, while Palpigrades, Schizomids, and Vinegaroons use theirs as sensory structures. Schizomids and Vinegaroons can also produce acetic acid at the base of their telsons, which they use as a chemical defense sprayed at attackers. Instead of a telson, spiders have spinnerets on their abdomen which they use to create silk. Some have no eyes; some have up to six. Some only have very simple eyes which detect light and shadow, while others have very advanced eyesight.

Arachnids are so diverse that it would be impossible for me to provide a general summary of their behavior here. Some lay eggs, some bear live young. Some have intensive courtship rituals. In most, mothers provide parental care, but in some this falls on the fathers. They can be predators, scavengers, carnivores, omnivores, herbivores, parasites, and everything in between.

Arachnids are one of the oldest living Classes of land animals. Xiphosurans first appeared in the Late Ordovician, with modern horseshoe crabs appearing in the Carboniferous and becoming more common in the Triassic. The spider-like Order Uraraneida first appeared in the Permian, with true spiders appearing in the Carboniferous. There’s a lot of talk about how horseshoe crabs predate the dinosaurs, and that’s true, but so do other arachnids… and most other arthropods as well!

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Round 2 - Arthropoda - Pycnogonida

(Sources - 1, 2, 3, 4)

Pycnogonida is a class containing one order: Pantopoda, which means “all feet.” A fitting name for creatures that seem to be made entirely of legs. Commonly called “Sea Spiders”, they are not spiders, nor are they arachnids, but are actually a sister group to all other living arthropods.

Pycnogonids live in most oceans. Most are tiny, living in relatively shallow water, though some can grow to be quite large in antarctic and deep waters. Some pycnogonids are so small that each of their muscles consists of a single cell. They have a proboscis which they use to suck nutrients from soft bodied invertebrates such as cnidarians, sponges, polychaetes, and bryozoans. They can also insert their proboscis into anemones, though this rarely kills the anemone. The pycnogonid digestive tract extends into their legs. They are segmented, with the first body segment (the cephalon) consisting of the proboscis, the ocular tubercle with up to 4 simple eyes, a pair of chelifores, a pair of palps, a pair of ovigers, and the first pair of walking legs. Ovigers are used for cleaning themselves, courtship, and caring for eggs and young. Nymphonidae is the only family where both the chelifores and palps (sensory organs) remain functional. In others, these limbs are reduced or absent, instead relying on a well-developed and flexible proboscis equipped with sensory bristles. Pycnogonids are usually comprised of eight walking legs, but the family Pycnogonidae includes species with ten, and the families Colossendeidae and Nymphonidae include species with up to twelve legs! While most species have up to 4 eyes, some deep-sea species lack them entirely. Pycnogonids do not have a traditional respiratory system, instead absorbing oxygen through their legs and diffusing it throughout their body via hemolymph. Their small, long, thin hearts beat vigorously at 90 to 180 beats per minute, creating substantial blood pressure. Their nervous system consists of a brain which is connected to two ventral nerve cords, which in turn connect to specific nerves. Like other arthropods, they molt their exoskeleton as they grow.

Pycnogonid reproduction involves external fertilization after a brief courtship involving the male stroking the larger female with his ovigers and receiving the eggs if she is responsive. The couple must adjust their position until the genital pores on their legs are perfectly aligned. Only males will care for eggs and young, and in some species only the males will have ovigers while the females do not, as these limbs are used mainly for carrying and cleaning the eggs. Larvae consist only of a head with chelifores, palps and ovigers. Extra segments and legs emerge as it grows into an adult. There are at least four different types of larvae. The typical protonymphon larva is most common, is free living and gradually turns into an adult. The encysted larva spends its larval days as a parasite, finding a host in a colony of polyps, burrowing into one, turning into a cyst, and not leaving the host until it has become a juvenile. The atypical protonymphon larva lives on or within a temporary host such as a clam or polychaete worm, does not encyst or otherwise harm their host, and leaves them as an adult. Lastly, the attaching larva hatches as an embryo and immediately clings to the legs of its father, only leaving once it has two or three pairs of its own walking legs.

The pycnogonid’s cerebral appendages are unique, not found anywhere else among arthropods, except in fossils like Anomalocaris. This could mean that pycnogonids are the last surviving (highly modified) members of an ancient stem group of arthropods that lived in Cambrian oceans.

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