ASTEROIDEA. 



291 



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 ambulacral groove, which contains, 

 outside the ossicles and beneath the soft skin (which in Ophiurids 

 possesses special calcareous plates), the nerve trunks, the peri- 

 hremal canals with the blood-vessels and the ambulacral trunks. 

 In the Qphiuridea the ambulacral groove is covered by the dermal 

 plates so tl'iat 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 papillae, 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 fi O Q R \ 



FIG. 23G. Skeletal plates of Asirnpecten Hempricliiii (after J. Muller). 

 Upon the veil- DR, Dorsal marginal plates; VR, ventral marginal plates, Ap, am- 

 tral surface bulacral ossicles; J-p, 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 JSc7iinoidea,vf}iere 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 articulated 

 together like vertebrae, with spaces between their lateral processes 

 for the passage of the vessel connecting the ampullae with the radial 

 vessel and the tube feet. The right and left pieces of each double row 

 are either (Ophiuridea) immovably connected by a suture, or are 



