The epidermis is composed of simple cuboidal to columnar epithelium of several cell types, best differentiated by electron microscopy. These include supporting cells, secretory cells, pigmented cells (chromatophores and iridophores), sensory cells, nerve cells, and coelomocytes. Supporting cells have microvilli along their apex and may have cilia. They have basally located oval nuclei and a prominent nucleolus. Secretory cells are nonciliated with microvilli present only at the apex. Although five types of secretory cells are recognized by electron microscopy, the features discernible by light microscopy are variations in vacuolar size, shape, and staining characteristics. This discerns essentially two cell types: mucous gland cells, with finely granular contents, and muriform cells filled with coarse spherules (Hyman 1955) (Figure 1.9). In some echinoderms, especially echinoids, epithelial cell types may be difficult to differentiate histologically. In areas surrounding papulae (eversions of the coelomic cavity used for respiration in Asteroidea), the epidermis may contain multicellular glands with specialized secretions. Sensory nerve cell bodies and their axons may be visible basally within the epidermis, often referred to as the subepidermal plexus (or the ectoneural nerve net). The sensory layer is thinnest near the papulae and thickest in the oral region where it forms a circumoral nerve ring. The sensory layer often forms a ring around the base of ossified appendages. Coelomocytes may be present in the epidermis due to their role in phagocytosis and excretion of waste products to the environment. Their features are described later. The inner body wall consists of a simple layer of squamous sparsely ciliated epithelial cells that line the coelomic cavity.
Table 1.1 Organs for histologic evaluation in Echinodermata.a
| Organ system | Organs | |
|---|---|---|
| Body wall/musculoskeletal | Cuticle, epidermis, dermis/mutable collagenous tissue, dermal ossicles, skeletal muscle, paxillaeb, pedicellariaeb | |
| Water vascular system | Madreporite, stone canal, circumoral ring canal, radial canal, tube feet | |
| Digestive | Alimentary canal | Mouth, esophagus, stomach, intestineb, rectumb |
| Pyloric and rectal cecae | Digestive tubules, pyloric duct, rectal duct | |
| Excretory | Heart, axial canal, axial hemal vessel, tube feet, papulae | |
| Circulatory | Heart, axial organ, axial hemal vessel, hyponeural (oral) hemal ring, gastric hemal ring, genital hemal ring | |
| Immune | Coelomocytes | |
| Respiratory | Papulae (gills), tube feet | |
| Nervous | Circumoral nerve ring, radial nerve, superficial and deep nerve nets | |
| Reproductive | Male | Testis, sperm ducts |
| Female | Ovary, oviduct | |
| Ovotestisb | Ovary, testis | |
| Special senses/organs | Eyespots, sensory tube feet | |
a Alternative names for organs are provided parenthetically, in italics.
b If present in a given species.
Figure 1.8 Low‐magnification image of the histology of the body wall of an (a) ochre sea star (Pisaster ochraceus), (b) white sea urchin, and (c) California giant sea cucumber. Hematoxylin & eosin (HE), 100×, 40×, 100×, respectively. D, dermis; E, epidermis; G, gonads; O or arrows, ossicles; P, papulae; Pd, pedicellaria; T, tube feet.
Table 1.2 Cuticular layers in echinoderms (Holland).
| Class | Layers present |
|---|---|
| Crinoidea | Fibrous outer layer (“fuzzy layer”) Granular inner layer |
| Echinoidea | Fibrous outer layer Granular middle layer Fibrous inner layer |
| Asteroidea | Fibrous outer layer Granular middle layer Fibrous inner layer |
| Ophiuroidea | Fibrous outer layer Granular middle layer Fibrous inner layer |
| Holothuroidea | Outer, rodlet layer Granular middle layer Fibrogranular inner layer |
The dermis is composed of mutable collagenous tissue and an endoskeleton composed of interconnected plates, which may be articulated to form a rigid structure. The endoskeleton is composed of magnesium‐rich calcium carbonate, as magnesian calcite, devoid of an organic matrix (Cavey and Märkel 1994). Magnesium, substituting for calcium, is a unique feature of the echinoderm skeleton relative to other invertebrates (Raup 1966). Endoskeletal plates are of various shapes and are often called ossicles. Ossicles are separated into small interdigitating sections that are adjoined by collagenous ligaments and skeletal muscle (Figure 1.10). They are typically adorned by tubercles that articulate with movable ossified appendages, such as spines or calcareous protuberances, pedicellariae, and sphaeridia. Specialized ossicles called paxillae are present on the aboral surface of certain sea star species and facilitate burrowing. In ophiuroids, ossicles form larger plates called shields and each arm segment (article) is composed of four shields, two lateral, one aboral and one oral, with the lateral shields having large spines. Echinoids lack a muscle layer in the body wall because skeletal plates are fused and immobile, although muscle tissue is still present at the sites 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.