THE ECHINODERMATA. 129 



composed of, 1. Series of plates which lie superficial to the 

 ambalacral canals, and are called ambulacral, being perforated 

 for the canals of the feet. 2. Plates which lie between these 

 series, and are termed interambulacral. 3. Five ocular and five 

 genital plates at the apical ends of the ambulacral and inter- 

 ambulacral series respectively. 



The larva? are pluteiform, and have a skeleton. 



2. The HOLOTHTJRIDEA (or Sea-slugs). The body is vermi- 

 form. The ambulacral tubes extend, as in the Echinidea, from 

 mouth to apex, but there are no regular ambulacral and interam- 

 bulacral plates. The genital organs have a single aperture 

 towards the anterior part of the body. 



The larval are vermiform, and have no skeleton. 



3. The ASTEEIDEA (or Star-fishes). The body is pentagonal, 

 or stellate, and depressed, the mouth being in the middle of one of 

 the flat sides. The ambulacral tubes extend only to the extreme 

 boundaries of the oral face of the discoidal body. Hard plates, or 

 ossicles, articulated together, constitute the skeleton, and a double 

 row of these lies on the deep side of the ambulacral canals, which, 

 on the superficial side, are covered only by the integument. 



The larvae are vermiform, and have no skeleton. 



4. The OPHIUEIDEA (or Sand-stars). The body is depressed, 

 and gives off five arms of a different structure from itself. 

 Each presents a central axis, formed by a chain of quadrate 

 ossicles, and, for each ossicle, a zone of four superficial plates, one 

 ventral, one dorsal, and two lateral. The ambulacral vessel lies 

 between the ventral plates and the quadrate ossicles, and only 

 extends to the summit of each arm. 



The larvae are pluteiform, and have a skeleton. 



5. The CRINOIDEA (or Feather-stars). The body is cup-like, 

 sometimes stalked and sometimes sessile. It gives off a 

 variable number of arms (usually five). The mouth is placed in 

 the middle of the cup, and the principal plates of the skeleton, 

 which are regularly articulated together, are developed on the 

 opposite side of the body to that on which the mouth is situated. 

 The arms are provided with articulated lateral processes or 

 pinnules, clothed in a broad membrane, in which the reproduc- 

 tive organs are placed. 



K 



