of articulation of the spines. In holothurorids, the ossicles are present but microscopic and are randomly distributed throughout the dermis. Some have paired specialized ossicles, the anchor and anchor plate, which assist in attaching species that lack tube feet to the substrate. A ring of well‐developed ossicles is present around the mouth and esophagus providing attachment sites for the buccal podia. Well‐developed longitudinal bands of smooth muscle are present along each ambulacrum.
Figure 1.9 Histology of the epidermis of a sunflower sea star (Pycnopodia helianthoides). Individual cell types are difficult to discern with light microscopy. The columnar epidermis (E) has occasional secretory cells (S). The subjacent dermis (D) contains many coelomocytes (C). 400×, Lee's methylene blue (LMB).
Figure 1.10 Low‐magnification image of the histology of a sunflower sea star ossicle demonstrating dermal, ligamentous, and muscular attachments. 200×, von Kossa.
Figure 1.11 Higher magnification image of the histology of an ochre sea star ossicle demonstrating the sclerocyte lattice (plastinated section). 400×, LMB.
Histologically, the endoskeleton consists of a three‐dimensional crystalline latticework, the stereom. Post decalcification, the calcite trabeculae are evident as clear spaces that may be artifactually collapsed. The fluid‐rich stroma that marginates trabeculae forms a honeycomb structure and contains sclerocytes that produce, modify, and envelop the skeleton (Figure 1.11). Sclerocytes are stellate mesenchymal cells that are typically in contact with trabeculae, and may be sparse within fully developed ossicles (Märkel and Roser 1983). In growing ossicles, sclerocytes form syncytia. Coelomocytes (discussed later) are common among the stroma, but may not necessarily be evenly distributed and can lead to a false impression of inflammation. Specialized phagocytes are capable of reabsorbing calcite from the ossicles (Ruppert et al. 2004). In echinoids, these are termed skeletoclastic cells and they are syncytial phagocytes that resemble osteoclasts (Cavey and Märkel 1994).
Figure 1.12 Histology of the base of a white sea urchin spine at the ball and socket joint. 400×, HE. M, muscle; L, ligament; T, test.
The osseous appendages have components that are similar to the body wall. All are covered in epidermis and contain an assemblage of dermal tissues described above. The echinoid spine consists of similar latticed endoskeleton with a central meshwork or hollow area surrounded by radiating longitudinal septae. The base of a spine adjoins to a tubercle of the test with ligaments of mutable collagenous tissue (i.e., the catch apparatus) encircled by bundles of smooth muscle cells (Figure 1.12). Distal spines of some urchins may be surrounded by a poison sac that has a collagenous connective tissue wall, and a lumen containing dissociated cells and debris (Cavey and Märkel 1994). Pedicellariae, present in Echinoidea and Asteroidea, clean the body surface and protect against sediment and small organisms. Microscopically, they consist of a stalk bearing a moveable head (Figure 1.13). Pedicellariae can be classified into a variety of types based on the size and shape of the head, and the number of jaws (i.e., tridentate, trifoliate, ophiocephalous, and globiferous). Most often, they have three elongate and distally narrowed jaws, each supported by a valve‐type ossicle, and supplied by adductor, abductor, and flexor muscles. The latter may be composed of smooth or striated myocytes. The stalk is supported by a rod‐shaped ossicle that may distally transition to a cavity filled with mucosubstances (Ghyoot et al. 1987). The epidermis is similar to that covering the test, but may be heavily ciliated along the stalk and inner jaws. Globiferous pedicellariae may carry venom sacs or epidermal glands on the inner jaws and these may be composed of more than one type of secretory epithelial cell (Ghyoot et al. 1994).
Figure 1.13 Histology of white sea urchin appendages including pedicellaria (P), spine (S), and tube foot (T). 100x. HE.
Dermal spaces between the endoskeleton are composed of fibrous connective tissue populated by stellate cells (Hyman 1955). A unique connective tissue termed mutable collagenous tissue is present in the body wall of all classes of echinoderms. Mutable collagenous tissue is controlled through a nonmuscular nervous system and can change its mechanical properties within one second to a few minutes from flaccid to rigid (Motokawa 1984, 2011; Wilkie 2002). The histologic features of mutable collagenous tissue (also called catch connective tissue) are not unlike dense irregular and regular connective tissues present in vertebrates. It is composed of individual collagen fibers with intervening ground substance that are arranged in perpendicular or parallel arrays depending on the species (Motokawa 1984). Interspersed among the fibers and ground substances are small numbers of immune cells (morula cells, coelomocytes). The function of this tissue varies by species and body wall structure. In holothuroids and asteroids, this tissue plays a significant role in overall body tone. In asteroids and echinoids, it plays a role in spine posture and prevents spine disarticulation. In crinoids, it controls the flexibility of the stalk (cirral) ligaments. In all species, it plays a significant role in autotomy (Motokawa 1984).
1.3.2 Water Vascular System
The water vascular system is a hydraulic system used for substrate adhesion, locomotion, and in some echinoderms prey manipulation. In many species tube feet also play an important role in respiration and excretion. It is composed of the madreporite, stone canal, circumoral ring canal, radial canal, ampullae, and tube feet (also called podia). The madreporite is a porous ossicle on the aboral surface of sea stars, sand dollars, and sea urchins and the oral surface of brittle stars. In sea cucumbers the madreporite is internal. Also known as the sieve plate, the madreporite functions as a valve which communicates with surrounding sea water. The madreporite and stone canal maintain fluid volume in the water vascular system (Ferguson 1990; Ferguson and Walker 1991). Coelomic fluid fills the water vascular system and is osmotically and ionically similar to sea water (Freire et al. 2011).
Figure 1.14 Histology of the madreporite (a) and stone canal (b) in a mottled star (Evasterias troschelii). 25×, 50×, HE. D, dermis; Dt, digestive tract; E, epidermis; G, gonad; M, madreporite; O, ossicles; S, stone canal.
The
