ECHINODERMATA 



91 



into a saclike stomach, the greater part of which can be pro- 

 truded from the mouth and inserted between the valves oi the 

 shells of mussels and similar animals on which the starfish 

 feeds. The deeper part of the stomach sends branches into 

 each arm, and these branches are connected with glandular 

 organs, the hepatic caeca, which assist in digestion. There 

 may or may not be an anus, connecting with the deeper part of 

 the stomach, in the central part of the aboral disc. The arms 

 of the common starfish are hollow, i.e. they possess a large 

 body-cavity, which, in 

 addition to the hepatic 

 caeca, contains also 

 the reproductive or- 

 gans or germ-glands, 

 of which there are 

 two in each arm, and 

 their ducts opening 

 to the outside in the 

 interradii. The am- 

 pulla? connected with 

 the inner ends of the 

 tube feet also lie in 

 the body-cavity, and 

 the feet pass out 

 through the openings 

 between the plates 

 of the oral skeleton. 

 The skeleton on the 



aboral Side Ot tile p 1Gm 8 _ Biachiolarian larva of the common starfish, Aste- 

 bodv Consists of an r ' as < v ent v al aspect; highly magnified. (Drawn from life 



by the author. 1 



irregular network of 



hard parts ; but on the oral side the calcareous plates are 

 arranged along the ambulacral groove like rafters in contact 

 with one another, and are called the ambulacral rafters ; there 

 is also more or less regularity in the arrangement of the skeletal 

 plates which connect these rafters with the aboral skeleton. 



The development of the Asteroidea, like that of all the 

 Echinodermata, is pretty complicated. The fertilized egg 

 develops into a bilaterally symmetrical, ciliated larva (Fig. 80). 



