ASTEROIDEA. 187 



there are tufts, along the sides of the abactinal surface of the 

 proximal parts of the arms. The gonad tubes are lined by a 

 simple epithelium which gives rise to the generative cells, and 

 the external openings are always on the dorsal surface except 

 in Asterina gibbosa and Asterina pancerii Gasco, in which they 

 open on the actinal surface, no doubt in correspondence with the 

 fact that these animals attach their ova to foreign objects. 



The composition of the gonads of a single tuft or of several tufts is often 

 a generic character, but sometimes both conditions are found in the same 

 genus (e.g. Echinaster). 



The gonads are really the peripheral parts of the generative 

 rachis. This structure consists of a cellular cord placed in the 

 wall of the aboral sinus on the dorsal side of the stomach. It 

 gives off a prolongation into the axial organ, and in each inter- 

 radius two cords proceed from it to the gonads. These cords 

 are accompanied by a prolongation of the aboral sinus, which 

 reaches as far as the gonads and surrounds them (p. 181). 



Except in those forms in which there are arrangements for 

 the care of the brood, external sexual differences are only occa- 

 sionally present, and are then usually confined to differences in 

 colour. 



A brood pouch is developed on the dorsal surface of the 

 Pterasteridae (see p. 192). In some of the Astropectinidae the 

 eggs pass into the interstices between the stalks of the paxilli 

 and there undergo their development. 



In Stichaster nutrix Studer describes the young as being at first in 

 outgrowths of the stomach where they undergo their early development, 

 and then as passing on to the edges of the mouth. In Asterias perrieri 

 Smith and other species the young are described as being attached to 

 the oral surface of the disc in the neighbourhood of the mouth, and there 

 undergoing their development ; the arms being slightly bent over them 

 for protection. In species of Di/i/nnfrrins similar phenomena appear 

 to occur. 



They are all marine and crawl upon the bottom of the sea. 

 They capture their food by means of their tube-feet and many 

 of them have the power of partially everting their stomach, the 

 inner surface of which is applied to their prey. 



Many of them have the power of autotomously severing their 

 arms from the disc and of regenerating arms so lost. The 

 power of regenerating lost parts is great in all members of the 



