Figure 1.23 Histology of gills (papulae) in a white urchin showing epidermal surface (E), supported by connective tissue (Ct), and a central lumen lined by coelomic epithelium (C). 200×, HE.

      In asteroids these evaginations of the body wall are called papulae. They can be branched and in species with paxillae, the papulae typically sit in the water‐filled branchial space beneath this umbrella‐shaped specialized surface structure. In regular echinoids there are five pairs of peristomial gills on the peristomial membrane, at the margin of each interambulacral plate, that likely provide gas exchange for the muscular apparatus of the lantern. These originate as evaginations from the peripharyngeal (lantern) coelom and have similar histologic features to asteroid papulae. Coelomic fluid is pumped to and from the peristomial gill lumen by the muscles and ossicles of Aristotle's lantern. In irregular echinoids, modified tube feet of the petaloids act as gills.

Histologically, peristomial gills, papulae, and petaloids are similar (Figure 1.23). They consist of a simple ciliated epidermis composed of supporting cells, a thin connective tissue dermis and a single layered coelomic epithelium lining a central canal. In echinoids, the coelomic epithelium forms small papillary invaginations into the central sinus when contracted. Pigmented cells and coelomocytes are often present, and their extrusion across the epidermis has given rise to the theory that gills have an excretory function (Cavey and Märkel 1994).

      Holothuroids have specialized podia near the oral cavity (buccal podia) and tube feet which, similar to other species, function as gills. The primary respiratory organ, which provides gas exchange to the coelomic viscera, is paired internal respiratory trees, which arise as diverticula from the wall of the cloaca. These diverticula form a highly branched system of blind‐ended tubes that contain sea water. Histologically, the structure of the respiratory tree is similar to papulae and peristomial gills. The internal surface is covered by a simple low cuboidal epithelium separated from the external coelomic epithelium by a very thin connective tissue dermis. Gas exchange occurs across the surface from sea water that is actively pumped into the respiratory tree from the cloaca.

      1.3.8 Nervous System

      The nervous system in echinoderms lacks ganglia, which are present in most other invertebrate species. The central nervous system in asteroids consists of a central circumoral ring and five radial nerves that extend within the center of the ambulacral groove to the tip of each ray. Each have a sensory and a motor component. The peripheral nervous system consists of the intraepithelial nerve nets previously described in the body wall. The sensory ectoneural nerve net extends along the epidermis and the motor hyponeural nerve net extends along the coelomic epithelial lining. These nerve nets are connected by neurons that cross the dermis. In Echinoidea, the ectoneural nerve system is the main component and consists of a circumoral nerve ring, radial nerves, podial nerves, and subepidermal nerve plexus. The radial nerves arise from the circumoral nerve ring and extend through the lantern and along the ambulacral plates, coursing between the radial canal and test. Radial nerves give rise to podial nerves that supply the tube feet. The hyponeural nerve system is a series of five radially positioned plaques of nervous tissue below the circumoral nerve ring. Some regular sea urchins have an entoneural nerve system consisting of a nerve ring around the periproct which gives rise to innervation of the gonads.

Histologically, the nerve ring and radial nerves of Asteroidea and Echinoidea have a distinct outer sensory layer, which communicates with the ectoneural system, and an inner hyponeural layer, which communicates with the motor components. The motor portions of the radial nerve innervate the ampullae, tube feet, and body wall musculature. In all other echinoderm classes other than Asteroidea, the circumoral nerve ring and radial nerve cords have been internalized. The ectoneural portions of the circumoral nerve ring and radial nerves are further isolated in a specialized epineural canal, which is lined by ciliated epithelium (Figure 1.24).

      1.3.9 Reproductive System

Asteroids and echinoids are dioecious and each has gonads suspended by mesenteries either as five paired structures within the ray or as five individual gonads suspended from the interradius. The gonad is connected by a short gonoduct to a gonopore opening at the base of the arms in asteroids or in the genital plates on the aboral surface of echinoids. Gonads have similar structures, whether ovary or testicle. They consist of an outer genital sac which has a thin connective tissue wall, an outer coelomic epithelial lining and an internal lining of germinal epithelium. Muscle fibers may be sparsely present within the connective tissue. Germ cells develop peripherally and mature centrally. Ovaries contain oogonia progressing to large well‐developed vitellogenic oocytes centrally. Testicles contain spermatogonia progressing to small round spermatozoa centrally (Figure 1.25). Sex may be histologically indiscernible in reproductively inactive or immature individuals. Somatic cells (nutritive phagocytes) are present in both sexes of echinoids and dominate during periods between and leading up to gonadogenesis (Figure 1.26) (Walker et al. 2007).

Holothuroids are dioecious but gonochoric and have an ovotestis rather than a separate ovary and testicle. The gonad is composed of a large tuft of finely branched tubules covered by thin layers of coelomic epithelium and muscle. It is lined by germinal epithelium that shows differentiation toward both ova and sperm. It is connected by a gonadal duct to a gonopore located immediately behind the mouth at the base of the buccal podia. This gonopore is lined by a simple columnar epithelium.

Figure 1.24 Histology of the ventral nerve cord (N) in a mottled star. 100×, LMB.

Figure 1.25 Histology of the ovary (a) in a Caribbean thorny star, and testicle (b) in a mottled star.

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