INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION ECDYSOZOA

PHYLUM ARTHROPODA

SUBPHYLUM CHELICERATA

SUPERCLASS PANTOPODA

CLASS PYCNOGONIDA

Exclusively marine Arthropods (over 640 species are known, with seven species found in the Black Sea). Most of them inhabit the sublittoral zone, though some species are also found at considerable depths (up to 5,000–7,000 m). The largest sizes are reached by deep-sea species of the genus Colossendeis, whose trunk length can reach 72 mm, and leg span up to 300 mm. However, the majority of species are significantly smaller. The smallest species, Anoplodactylus pygmeus, has a trunk length of 0.8 mm with a leg length of 1.4 mm.

Anatomy. The body of sea spiders is extremely small compared to their limbs and consists of a 7- to 9-segmented cephalothorax and a rudimentary abdomen. The first four segments of the cephalothorax fuse to form the HEAD, or ocular region, while the others may either remain distinct or fuse with one another and with the head region (Fig. 307).

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Fig. 307. Sea spiders (from Turpayeva):

a - Nymphon distensum; b - Decalopoda australis: 1 - palps; 2 - chelifores;

3 - proboscis; 4 - walking legs; 5 - cephalothorax; 6 - abdomen

The head region bears a cylindrical or ovoid proboscis, four simple ocelli, and four pairs of appendages. The first pair consists of chelate chelicerae (chelifores) used for capturing prey; the second pair comprises tentacle-like palps; and the third consists of ovigerous legs, which are mostly well-developed in males. The fourth pair, like the appendages of the thoracic region, Functions as walking legs (typically there are four pairs, though five or six may occur). Leg length is often 10 to 20 times the length of the trunk, giving the impression that the animal consists of little more than legs (hence the name, from the Greek *pan* - all, *podos* - FOOT).

The body of pantopods is covered with a chitinous cuticle, beneath which lies an epithelium rich in various glands, including those that function as excretory Organs.

The Digestive System begins with the Mouth opening, which leads into the proboscis cavity lined with cuticle. Inside the proboscis is a large muscular Pharynx that acts as a pump, allowing the animal to suck up liquid food. At the rear of the pharynx, numerous intersecting hairs form a filter to strain the food; solid particles are trapped by this filter and subsequently expelled outward (Fig. 308). The midgut is short and forms lateral outgrowths (diverticula) that extend into the limbs and reach their tips. A short hindgut terminates in the anus.

Fig. 308. Longitudinal section through the proboscis of the sea spider Phoxichilus vulgaris (from Turpayeva):

a - proboscis with filtering apparatus; b - section of the filtering apparatus

Adult pantopods feed on the soft Tissues of various marine invertebrates, such as hydroids, coral polyps, jellyfish, Sponges, and bryozoans.

The Circulatory system consists of a Heart resembling a U-shaped tube with paired ostia; Blood Vessels are absent. Colorless hemolymph circulates through a system of lacunae.

The Nervous system is represented by supra- and subesophageal ganglia connected by connectives, and a ventral nerve cord. In some species, a concentration of the ventral cord ganglia is observed. Additionally, a nerve ring with three ganglionic thickenings, connected to the Brain, is located at the anterior end of the proboscis. A nerve plexus extends from this ring along the pharynx.

Sense Organs are generally poorly developed. Aside from the aforementioned ocelli, which distinguish only light intensity and direction (and likely the movement of surrounding objects), pantopods possess sensory hairs and bristles scattered across the body.

Reproductive System. Sea spiders are dioecious animals, often exhibiting pronounced Sexual Dimorphism, primarily in The Structure of the ovigerous legs. The Gonads are paired, located on both sides and above the intestine. They appear as elongated tubes extending into the limbs, where the maturation of Germ Cells takes place; these cells are then released outward through the second segment of the legs.

Reproduction. The male winds the fertilized eggs onto the third

pair of limbs, where they are glued together by secretions from specialized glands. The male carries the eggs until the larvae hatch. Development proceeds with metamorphosis. A protonymphon larva emerges from the egg. It has a short body with a proboscis, three-segmented chelifores, and two pairs of three-segmented legs (Fig. 309). The larvae of some species develop on the male's body without leaving the egg mass, but in most sea spiders, the larva leaves the male and shifts to a parasitic lifestyle on hydrozoan polyps, sea anemones, within the mantle cavity of bivalve Mollusks, on the ambulacral grooves of sea stars, etc.

Fig. 309. Larva of the pantopod Chaetonymphon (after Turpaeva)