132 



Echinoderma. 



its tip. At the distal end of each groove, there is an unpaired 

 filiform structure, which bears on its lower surface close to the base, 

 several small red eye-spots: since the tips of the arms are 

 curved upwards, the eyes look up in spite of their position on the 

 ventral surface. The genital apertures usually occur on the 

 dorsal surface of the disc ; two or more tiny openings in each inter- 

 radius. The body-wall is much calcified, especially on the ventral 

 surface; a series of yoke-like calcareous plates movably jointed 



A B 



Fig. 87. Diagrammatic figures explaining the structure of a Starfish. A oral, 

 B aboral view ; in B some of the internal organs are figured. 61 caecum of the stomach, 

 Tc genital gland, V genital pore, m madreporite, o mouth, s tube-feet, t stomach, o eye- 

 spots. — Orig. 



together, and each consisting of a pair of closely-connected calcifica- 

 tions, roofs in the ambulacral groove : the radial water-vessel and 

 the radial nerve lie ventral to these ossicles. The upper side of the 

 body is less strongly calcified; numerous delicate thin-walled out- 

 growths of the body-wall, which may be regarded as gills, project 

 from it : they are not connected with the vascular system, nor do 

 they contain blood-vessels. Dorsally, at the edges of the arms, and 

 ventrally, as far as the margins of the ambulacral grooves, numerous 

 movable or fixed spines may occur together with pedicellarise 

 which are sessile, or provided with short stalks. 



In order to ingest large animals, Lamellibranchs, Sea-urchins, and 

 the like, the Starfish evert the stomach through the mouth, so as to 

 cover the prey, which is killed by the action of the digestive juices, 

 and its soft parts dissolved and absorbed. Smaller animals are 

 received entire into the non-everted stomach, the indigestible portions 



