. Elementary text-book of zoology. Zoology. ASTEROIDEA. which reach from the mouth to the end of the arms, and are articulated together like vertebrae. The skeleton of the Asteroidea is distinguished from the globular or flattened shell of the Echinoidea by the fact that the ambulacra! and interambulacral plates are confined to the ventral surface, and that on the outer side of the former there is a deep ambidacral yroove, which contains, outside the ossicles and beneath the soft skin (which in Ophiurids possesses special calcareous plates), the nerve trunks, the peri- haemal canals with the b
. Elementary text-book of zoology. Zoology. ASTEROIDEA. which reach from the mouth to the end of the arms, and are articulated together like vertebrae. The skeleton of the Asteroidea is distinguished from the globular or flattened shell of the Echinoidea by the fact that the ambulacra! and interambulacral plates are confined to the ventral surface, and that on the outer side of the former there is a deep ambidacral yroove, which contains, outside the ossicles and beneath the soft skin (which in Ophiurids possesses special calcareous plates), the nerve trunks, the peri- haemal canals with the blood-vessels and the ambulacral trunks. In the Ophiuridea the ambulacral groove is covered by the dermal plates so that the ambulacral feet project at the sides of the arms. Upon the dorsal surface the dermal skeleton appears leathery; it is, however, as a rule, filled with small calcareous plates, on which are placed spines, protuberances, and papilla?, constituting a covering of a most varied kind. At the mar- gin in the dorsal integu- ment there is usually a row of larger cal- careous plates (superior mar- ginal plates) (i* .T O f? \ ' ' FIG. 236.—Skeletal plates of Astropecten Hemprichtii (after J. Miiller). Upon the veil- DR, Dorsal marginal plates; VR, ventral marginal plates, Ap, am- ,i _ s bulacral ossicles ; Jp, intermediate interambulacral plates ; Adp, anterior adambulacral plates forming an angle of the mouth. we can distin- guish, in addition to the internally placed ambulacral ossicles, inferior marginal ossicles (fig. 236, VR), also the adambulacral plates (A dp), and the intermediate interambulacral plates (Jp). The two last corre- spond to the interambulacral plates of the Echinoidea, where they occur as two or more rows, which are united along the whole length of the inter-radius : in the Asteroidea, however, they separate from one another at an angle, and are disposed along the opposed sides of ad- jacent arms. The ambulacral ossicles are calcareous bodies
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
