5*3 



CLASS ASTEROIDEA. 



Echinozoa with bodies flattened dor so-ventr ally, pentagonal in outline or 

 prolonged into arm-like extensions, usually five in number. Arms with a 

 ventral ambulacral ftirrow lodging the tube-feet which end in discs. The 

 madreporite lies dor sally in an inter radius. Caecal extensions of the digestive 

 tract extend into the arms. The anus is dorsal. The genital glands are at 

 the base of the arms and extend more or less into them. Pedicellariae present 

 universally. 



The ectoderm cells develope a cuticle and are ciliated. There are 

 among them, in some instances at least, gland and sense cells, and a sub- 

 ectodermic plexus of nerve fibres and ganglion cells. A system of connected 

 spaces ramifies through the connective tissue of the integument and 

 appears to communicate with the peri-haemal spaces. The apical system 

 of plates is well defined in the young Asteroid and includes a dorso- 

 central, five basal and radial plates. The radials appear late, and are 

 preceded in point of time by the terminal plate at the apex of the growing 

 arm. This plate is large, and, as in Ophiuroidea, moves outwards with the 

 growth of the arm. It supports the ocular tentacle or first azygos tube- 

 foot, and is connected to the primary radial by a linear series of plates, of 

 which the last formed is, as in Ophiuroidea, next to it. The basals x appear 

 early, and the water-pore (madreporite) is frequently but by no means 

 invariably connected to one of the circle. Under-basals are present in the 

 young Asterina gibbosa, and persist in some adult forms, e. g. Zoroaster 

 fidgens, many Goniasteridae, &c. The apical system is generally not 

 traceable in the adult, but it persists in Zoroaster ftdgens, many Gonias- 

 teridae, &c. The oral system is represented by five plates which constitute 

 the odontophores of the adult. They are inconspicuous, and, as a rule, 

 hidden by the mouth plates, but in some deep-water species appear in part 

 on the surface. The calcareous structures of the body wall or perisome 

 occur as (i) granular ossicles ; (2) a network of ossicles, some of which 

 bear spines ; (3) plates variable in size, and mutual closeness ; and (4) 

 paxillae or plates bearing spines which spread out at their summits into 

 a number of radiating processes (Astropectinidae). There is often a special 

 series of dorsal and ventral marginal plates bordering the edges of the 

 arms. Spines, when present, are relatively short : they are fixed or free, 

 the latter being especially the case with those at the sides of the ambu- 

 lacral grooves over which they can be closed. The pedicellariae are either 

 sessile or stalked (Asteriadae), but the stalk never contains calcareous 

 supports. The valves or blades are two in number except in Luidia, 



1 The basals are commonly termed genitals, but they do not appear to have any connection with 

 the genital apertures as they have in the majority of Echinoids. 



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