Invertebrate Histology. Группа авторов

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Invertebrate Histology - Группа авторов

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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 counterclockwise (when viewed from a dorsal or aboral aspect), and this segment is sometimes referred to as the stomach, or small or inferior intestine. Most echinoids have a slender extension of the intestine that accompanies this first coil at its inner border, termed the siphon, and it is believed to facilitate extraction of water from food. Then, the intestine turns back on itself and courses dorsally and clockwise to form a second coil, and this segment is sometimes referred to as the large or superior intestine. Finally, the terminal intestine forms the rectum that ascends to the interior of the periproct and forms the anus.

Photo depicts the histology of the small intestine of a white urchin. Photo depicts the low-magnification histology of anatomy of Aristotle's lantern in a white urchin. Inset shows closer view of interpyramidal muscle.

      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).

Photo depicts an axial gland in a white urchin.

      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,

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