Sea spider

Sea spiders are marine arthropods of the class Pycnogonida, hence they are also called pycnogonids. The class includes the only extant order Pantopoda, alongside a few fossil species which could trace back to the early or mid-Paleozoic.
They are cosmopolitan, found in oceans around the world. The over 1,300 known species have leg spans ranging from to over. Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in the Antarctic region and in deep waters.
Despite their name and slight resemblance, "sea spiders" are not spiders, nor even arachnids. While some literature around the 2000s suggests they may be a sister group to all other living arthropods, their traditional classification as a member of chelicerates alongside horseshoe crabs and arachnids has regained wide support in subsequent studies.

Morphology

Many sea spiders are recognised by their enormous walking legs in contrast to a reduced body region, resulting into the so-called "all legs" or "no body" appearance. The body segments are generally interpreted as three main sections : cephalon, trunk and abdomen. However, the definition of cephalon and trunk might differ between literature, and some studies might follow a prosoma –opisthosoma definition, aligning to the tagmosis of other chelicerates. The exoskeleton of the body is tube-like, lacking the dorsoventral division seen in most other arthropods.
The cephalon is formed by the fusion of ocular somite and four anterior segments behind it. It consists of an anterior proboscis, a dorsal ocular tubercle with eyes, and up to four pairs of appendages. Although some literature might consider the segment carrying the first walking leg to be part of the trunk, it is completely fused to the remaining head section to form a single cephalic tagma. The proboscis has three-fold symmetry, terminating with a typically Y-shaped mouth. It usually has fairly limited dorsoventral and lateral movement. However, in those species that have reduced chelifores and palps, the proboscis is well developed and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. The proboscis is unique to pycnogonids, and its exact homology with other arthropod mouthparts is enigmatic, as well as its relationship with the absence of labrum in pycnogonid itself. The ocular tubercle has up to two pairs of simple eyes on it, though sometimes the eyes can be reduced or missing, especially among species living in the deep oceans. All of the eyes are median eyes in origin, homologous to the median ocelli of other arthropods, while the lateral eyes found in most other arthropods are completely absent.
In adult pycnogonids, the chelifores, palps and ovigers are variably reduced or absent, depending on taxa and sometimes sex. Nymphonidae is the only family where all of three pairs are always functional. The ovigers can be reduced or missing in females, but are present in almost all males. In a functional condition, the chelifores terminate with a pincer formed by two segments, like the chelicerae of most other chelicerates. The scape behind the pincer is usually unsegmented, but could be bisegmented in some species, resulting into a total of three or four chelifore segments. The palps and ovigers have up to 9 and 10 segments respectively, but can have fewer even when in a functional condition. The palps are rather featureless and never have claws in adult Pantopoda, while the ovigers may or may not possess a terminal claw and rows of specialised spines on its curved distal segments. The chelifores are used for feeding and the palps are used for sensing and manipulating food items, while the ovigers are used for cleaning themselves, with the additional function of carrying offspring in males.
The leg-bearing somites are either segmented or fused to each other, carrying the walking legs via a series of lateral processes. In most species, the legs are much larger than the body in both length and volume, only being shorter and more slender than the body in Rhynchothoracidae. Each leg is typically composed of eight tubular segments, commonly known as coxa 1, 2 and 3, femur, tibia 1 and 2, tarsus, and propodus. This terminology, with three coxae, no trochanter, and using the term "propodus", is unusual for arthropods. However, based on muscular system and serial homology to the podomeres of other chelicerates, they are most likely coxa, trochanter, prefemur/basifemur, postfemur/telofemur, patella, tibia and two tarsomeres in origin. The leg segmentation of Paleozoic taxa is a bit different, noticeably they have annulated coxa 1 and are further divided into two types: one with flattened distal segments and first leg pair with one less segment than the other leg pairs, and another one with an immobile joint between the apparently fourth and fifth segment which altogether might represent a divided femur. Each leg terminates with a main claw, which may or may not have a pair of auxiliary claws on its base. Most of the joints move vertically, except the joint between coxa 1–2 which provide lateral mobility, and the joint between tarsus and propodus did not have muscles, just like the subdivided tarsus of other arthropods. Adults usually have eight legs in total, but in a few species, adults have five to six pairs. These are known as polymerous species, which are distributed among six genera in the families Pycnogonidae, Colossendeidae and Nymphonidae.
Several alternatives had been proposed for the position homology of pycnogonid appendages, such as chelifores being protocerebral/homologous to the labrum or ovigers being duplicated palps. Conclusively, the classic, morphology-based one-by-one alignment to the prosomal appendages of other chelicerates was confirmed by both neuroanatomic and genetic evidences. Noticeably, the order of pycnogonid leg pairs are mismatched to those of other chelicerates, starting from the ovigers which are homologous to the first leg pair of arachnids. While the fourth walking leg pair was considered aligned to the variably reduced first opisthosomal segment of euchelicerates, the origin of the additional fifth and sixth leg pairs in the polymerous species are still enigmatic. Together with the cephalic position of the first walking legs, the anterior and posterior boundary of pycnogonid leg pairs are not aligned to those of euchelicerate prosoma and opisthosoma, nor the cephalon and trunk of pycnogonid itself.

	0	1	2	3	4	5	6	7
Euchelicerates	labrum	chelicerae	pedipalps	leg 1	leg 2	leg 3	leg 4	chilarium in horseshoe crabs, appendage absent in arachnids
Pycnogonids	?	chelifores	palps	ovigers	leg 1	leg 2	leg 3	leg 4

The abdomen does not have any appendages. In Pantopoda it is also called the anal tubercle, which is always unsegmented, highly reduced and almost vestigial, simply terminated by the anus. It is considered to be a remnant of opisthosoma/trunk of other chelicerates, but it is unknown which somite it actually aligned to. So far only Paleozoic species have segmented abdomens, with some of them even terminated by a long telson.

Internal anatomy and physiology

A striking feature of pycnogonid anatomy is the distribution of their digestive and reproductive systems. The pharynx inside the proboscis is lined by dense setae, which is possibly related to their feeding behaviour. A pair of gonads is located dorsally in relation to the digestive tract, but the majority of these organs are branched diverticula throughout the legs because the body is too small to accommodate all of them alone. The midgut diverticula are very long, usually reaching beyond the femur of each leg, except in Rhynchothoracidae where they only reach coxa 1. Some species have additional branches or irregular pouches on the diverticula. There is also a pair of anterior diverticula which corresponds to the chelifores or is inserted into the proboscis in some chelifores-less species. The palps and ovigers never contain diverticula, although some might possess a pair of small diverticula near the bases of these appendages. The gonad diverticula reach each femur and open via a gonopore located at coxa 2. The structure and number of the gonopores might differ between sexes. In males, the femur or both femur and tibia 1 possess cement glands.
Pycnogonids do not require a traditional respiratory system. Instead, gasses are absorbed by the legs via the non-calcareous, porous exoskeleton and transferred through the body by diffusion. The morphology of pycnogonid creates an efficient surface-area-to-volume ratio for respiration to occur through direct diffusion. Oxygen is absorbed by the legs and is transported via the hemolymph to the rest of the body with an open circulatory system. The small, long, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure. The beating of the heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs. Hemolymph circulation in the legs is mostly driven by the peristaltic movement of the gut diverticula that extend into every leg, a process called gut peristalsis. In the case of taxa without a heart, the whole circulatory system is presumed to be solely maintained by gut peristalsis.
The central nervous system of pycnogonid largely retains a segmented ladder-like structure. It consists of a dorsal brain and a pair of ventral nerve cords, intercepted by the esophagus. The former is a fusion of the first and second brain segments —protocererum and deutocerebrum—corresponding to the eyes/ocular somite and chelifores/somite 1 respectively. The whole section was rotated during pycnogonid evolution, as the protocerebrum went upward and the deutocerebrum shifted forward. The third commissure is established inferior to the esophagus. This third brain segment, or tritocerebrum, is fused to the oviger/somite 3 ganglia instead, which is followed up by the final ovigeral somata in the protonymphon larva of Pycnogonum litorale. A series of leg ganglia develop as molts progress, with incorporation of the first leg ganglia into the subesophageal ganglia in certain taxa. The leg ganglia might shift anteriorly or even cluster together, but are never highly fused into the ring-like synganglion of other chelicerates. The abdominal ganglia are vestigal, absorbed by the preceding leg ganglia during juvenile development.