The ampullae are elongate sacs that may be divided from the radial canal by a valve and have circular and longitudinal layers of muscle fibers. The podia consist of a stalk and terminal disc. They have layers similar to the body wall – an outer epidermis, middle connective tissue layer, and interior coelomic epithelial lining (Hyman 1955). The epidermis of the podia contains larger numbers of secretory cells than the rest of the body. The epidermis of the disc becomes thickened and is composed of ciliated columnar cells, larger numbers of secretory cells and neurosensory cells with a more prominent subepidermal nerve plexus, and is supplied by many subepidermal glands that may include mucous cells and granular secretory cells (Nichols 1961). Subjacent to the glands, the disc may be supported by latticed endoskeletal fragments. In addition to the subepidermal nerve plexus, a podial nerve may be evident coursing longitudinally on one side of the stalk. The stalk consists mainly of a cylinder of collagenous connective tissue (potentially divided into outer thicker longitudinal and inner thinner circular layers), supported by calcareous spicules (Figure 1.16). There is a central lumen (or hydrocoel) lined by a similar myoepithelium as observed throughout the water vascular system. Podia also have thick longitudinal retractor muscles which can contract the podia and push coelomic fluid back into the ampullae.
Figure 1.15 Histology of the water vascular (radial) canal in a white urchin. 200×, HE.
Figure 1.16 Histology of a tube foot in a mottled star. 25×, HE. C, connective tissue; D, disc; E, epidermis; H, hydrocoel; M, muscle; O, ossicle; S, stalk.
1.3.3 Digestive System
Echinoderms are a diverse group of animals with different nutritional strategies reflected in their digestive tracts. All consist of a simple tubular structure extending from the mouth to the anus with varying modifications that aid in digestion. In asteroids, the alimentary canal consists of a mouth, esophagus, stomach (cardiac, pyloric), intestine, and rectum. The mouth is at the center of the peristomial membrane and is separated by a muscular sphincter from the short esophagus and a more complex stomach. The cardiac portion of the stomach is large and has 10 distinct pouch‐like structures (radial pouches). Five of the pouches extend into the lumen of the arm from the disc and are attached to the ambulacral ossicles by muscle and dense connective tissue. A pair of gastric ligaments anchors the esophagus and permits retraction of the cardiac stomach in species that evert it during feeding. Above the radial pouches are five interradial pouches that eventually transition into the pyloric portion of the stomach. The pyloric stomach is smaller, flattened, and “star shaped” with five ducts that each extend into the central coelomic cavity of each ray and connect with the heavily branched pyloric cecae. The upper portion of the stomach tapers to form a short intestine that can have its own series of short blind sacs (intestinal cecae). The intestine connects to the short rectum and anus (Leake 1975; Ruppert et al. 2004).
The gastrodermis of the asteroid cardiac stomach is a pseudostratified columnar epithelium (Figure 1.17a). These cells lie on a basal lamina and basiepithelial nerve plexus with a connective tissue wall and outer coelomic epithelial liming. Circular and longitudinal muscle layers are interwoven into the coelomic lining. The gastrodermis is composed of supporting cells, secretory cells, and two types of coelomocytes. Supporting cells have a single cilium and numerous long microvilli. Secretory cells have no cilia and are either mucous or glandular in type. Two types of coelomocytes are normally seen in the gastrodermis and are found at all levels of the gut wall. The gastrodermis of the pyloric stomach is similar to the cardiac. Both the gastrodermal lining and the entire wall are thinner in the pyloric stomach due to reduced presence/thickness of the basiepithelial nerve layers, connective and muscle tissue layers (Figure 1.17b).
Figure 1.17 Histology of the cardiac (a) stomach in a mottled star (100×, HE) and pyloric stomach (b) of a mottled star (200×, HE).
Figure 1.18 Histology of the pyloric cecae of a mottled star. 25×, HE.
The pyloric and intestinal cecae are only present in asteroids. They are foliate structures created by extensive diverticula, which extend laterally from a medial duct. The diverticula are further divided into secondary chambers that are arranged parallel to the median duct. The lining of the pyloric cecae consists of very tall ciliated supporting cells and glandular secretory cells (mucous and zymogen cells) which are most abundant in the distal chambers of the pyloric cecae. Storage cells, cells containing large lipid vacuoles and polysaccharide and glycogen laden vacuoles, are more abundant distally (Hyman 1955; Leake 1975) (Figure 1.18).
The gastrodermis of the intestine and intestinal cecae is a ciliated pseudostratified columnar epithelium that in some areas may be compressed into a simple columnar epithelium and appear similar to the lining of the stomach. The epithelium is composed of supporting cells and two types of mucous secretory cells. The muscle, connective tissue, and nervous components are poorly developed in the intestinal cecal wall. The gastrodermis of the rectum and anus are identical and consist of a pseudostratified columnar epithelium composed predominantly of monociliated supporting cells attached to a basal lamina. The basiepithelial nerve plexus is reduced to absent. The connective tissue layer is thicker than in the intestine and pyloric cecae (5–10 μm thick) and is composed of thin elastic fibers (Hyman 1955). In Ophiurioidea, the digestive system is composed of a mouth, esophagus, stomach, rectum, and anus but lacks an intestinal tract and all components have histologic features similar to those described in asteroids.
In Echinoidea there is a mouth and a unique masticatory apparatus, Aristotle's lantern, followed by the esophagus, intestine, rectum, and anus. Aristotle's lantern is a pentamerous cone made of 40 ossicles including five teeth, adjoined by muscles and confined by coelomic membranes. At the ventrum of the lantern, the mouth is surrounded by a peristomial membrane, composed of mutable collagenous tissue covered in epidermis. Food passes through the mouth into a short pentagonal pharynx suspended in the center of the lantern. The pharynx transitions to esophagus at the top of the lantern. The esophagus ascends and then loops back as intestine. A blind pouch, variably referred to as stomach or cecum, may be present at the junction of esophagus and intestine. The intestine coils along the inside of the test, suspended by peritoneal membranes (i.e., mesenteries). The first nearly complete coil courses