Histologically, the echinoid digestive tract has layers similar to other echinoderms. The epithelial lining is composed of tall columnar ciliated epithelial cells termed enterocytes, some of which bear microvilli, and others that may be distinguished as mucous cells (Figure 1.19). Similar to the epidermis, there is a subtle nervous layer at the base of enterocytes. Subjacent to this is a thin layer of connective tissue, followed by a thinner layer of muscle cells, typically arranged in a circular pattern relative to the lumen. The outer layer consists of a simple layer of flagellated cuboidal epithelial cells, as found on coelomic surfaces of other viscera. Glandular crypts may form where shortened enterocytes segmentally invaginate. Oral (small) and aboral (large) intestine may be histologically distinguished by differential presence of glands, villi, thickness, or prominence of microvilli (Work n.d.; Francis‐Floyd et al. 2020). The siphon is histologically similar to the small intestine, only of smaller diameter. Histologic sections through the lantern typically feature major ossicles (i.e., the pyramids, compass, and rotula), teeth, interpyramidal (or comminator) muscles, the pharynx, peristomial membrane, the circumoral nerve ring, and sometimes gill at the lateral margin of the lantern (Figure 1.20). The central cavity of the lantern coelom that surrounds the pharynx reflects between folds of interpyramidal muscle. Its myocytes are arranged into rows along a thin connective tissue septum and are covered by a layer of squamous and ciliated adluminal cells (Märkel et al. 1990). The protractor and retractor muscles exterior to the base of the lantern are instead arranged into fascicles within connective tissue matrix (Ziegler et al. 2012).
Figure 1.19 Histology of the large intestine of a white urchin. 200×, HE.
Figure 1.20 Low‐magnification histology of anatomy of Aristotle's lantern in a white urchin. Inset shows closer view of interpyramidal muscle. 20×, HE. I, interpyramidal muscle; M, mouth; P, pharynx; T, teeth.
In Holothuroidea, there is a mouth, pharynx (calcareous ring), esophagus, stomach, anterior and posterior intestine, and cloaca. The mouth is at the center of a buccal membrane and is surrounded by a muscular sphincter. This leads to a short pharynx enclosed in a ring of ossicles. The stomach may not be present in some species and is generally not as well defined as in Asteroidea. The pharynx and stomach have a tall columnar epithelial lining composed of supporting and glandular cells showing mucous cell differentiation. Both have an internal cuticular lining unlike other species. The intestinal tract in holothuroids is extensive and is the primary site of digestion. The anterior portion (small intestine) has an extensive associated vascular system. It is lined by tall ciliated epithelial cells with prominent glandular differentiation and has a thin muscular wall. The posterior portion (large intestine) has a thinner epithelium with more prominent mucous cell differentiation. The digestive system of Crinoidea is confined to the disc and consists of a mouth, esophagus, intestine, rectum, and anus (anal cone) (Ruppert et al. 2004). Histology is similar to previously described echinoderm species.
1.3.4 Excretory System
In most echinoderms nitrogen excretion is primarily in the form of ammonia, which can diffuse across thin portions of the body wall at the papulae and tube feet. Coelomocytes facilitate excretion of other nitrogen‐containing metabolites (urates) and particulates through pinocytosis. Coelomocytes accumulate waste material internally and carry these accumulations to the gills, tube feet, and axial organ for either disposal or storage. Crinoids have no specialized excretory organs but are believed to be ammonotelic.
1.3.5 Circulatory System (Hemal System or Axial Complex)
In Asteroidea, hemal sinuses at the margins of the gut drain to the hemal ring that surrounds the base of the esophagus. The axial duct arises from the hemal ring, courses with the stone canal to the dorsal/aboral body, and enters the axial complex beneath the madreporite. The axial organ is adjoined by the axial duct, forming a junction between the coloemic cavity, water vascular system, and hemal system. The exact role of the axial complex is currently undetermined. Hypotheses include roles in respiration, excretion, and waste disposal, an immune organ, a gland of unknown purpose, a coelomocyte‐producing organ, a site of cell degradation, or a heart (Ziegler et al. 2009).
Histologically, hemal sinuses (or lacunae) have a wall of connective tissue that is lined exteriorly by coelomic epithelium. Muscle fibers in circular or longitudinal profile are scant throughout the wall. There is no inner lining or endothelium. Pigmented cells presumed to be phagocytes laden with melanin are often within vessels of the hemal system, and these may increase with age. The axial gland (or axial organ) is associated with the stone canal and consists of meshwork of connective tissue populated by coelomocytes (Figure 1.21) (Ziegler et al. 2009). Invaginations of the coelomic lining and lacunae penetrate the hemal sinuses. Cells containing melanin pigment are often within the stroma (Bachmann and Goldschmid 1978). The external surface of the axial gland is lined by coelomic epithelium.
Figure 1.21 Axial gland in a white urchin. 400×, HE.
Five pairs of Tiedemann's bodies adorn the hemal ring at the interradial areas in Asteroidea and the dorsal lantern in Echinoidea. In echinoidea they are formed where the coelomic lining of the dorsal lantern engages with evaginations from the radial canals (Cavey and Märkel 1994). Histologically, these are similar to the axial organ (Figure 1.22). A meshwork of connective tissue is permeated by canaliculi lined by coelomic epithelium. Coelomocytes and pigmented cells are also similarly frequent.
1.3.6 Immune System
Coelomocytes exist within the fluid of the coelomic cavity, water vascular system, and hemal system, and are seen throughout all tissues of the body (Holland et al. 1965). They play diverse roles including nutrient delivery, waste excretion, phagocytosis, immune response, clotting, and wound healing. Nine different coelomocyte types have been described in sea stars (Kanungo 1984) but by light microscopy these cell types are not discernible. Some discerning features are evident using electron microscopy. Coelomocytes in echinoids include phagocytes (amoebocytes), spherule cells, and vibratile cells (Cavey and Märkel 1994), best distinguished by cytology. Phagocytes are the most abundant and may have cytoplasmic foreign material. Vibratile cells are small, round, and flagellated. Coelomocytes with eccentric nuclei and cytoplasmic inclusions are nonphagocytic and often referred to as granular or spherule cells, which are further named according to the color of their inclusions (i.e., red or colorless). Red spherule cells contain echinochrome, a red naphthaquinone pigment. In holothuroids there are six different types of coelomocytes recognized, including morula cells, amoebocytes,