PHYSIOLOGY OF THE INVERTEBRATA. 415 



Before the formation of the mouth, two lobe-like bodies are 

 formed from the upper part of the alimentary canal. These 

 lobes ultimately separate and form two distinct cavities. 

 These develop into two water-tubes which elongate until 

 they surround the alimentary canal, extending on the other 

 side of it beyond the mouth where they join, giving rise to 

 a Y-shaped canal. (t) The ventral ridge containing the 

 band of cilia becomes prolongated into processes of various 

 shapes. These processes are arranged with close regard to 

 bilateral symmetry {Bvpinnaria and Brachiolaria, or bilateral 

 larval stage). (/) At this point the body of the future 

 starfish begins to develop from the larva. "On one of the 

 branches of the united water-tubes the feet or tentacula are 

 produced as a series of lobes, while on the opposite branch 

 of the water- vascular canal many small calcareous rods are 

 elaborated. These rods afterwards form a regular network, 

 and indicate the dorsal side of the young Astcrias." {g) At 

 this stage the larva Brachiolaria shrinks and drops to the 

 bottom of the water, where it fixes itself by means of its short 

 arms. (A) That portion of the larva which is more developed 

 into the true starfish form than the remaining portion, now 

 absorbs the latter and acquires a conical and disc-like form, 

 with a crenulated edge. In this stage the organism remains 

 for two or three years. Then the rays or arms lengthen and 

 the mature form is assumed. 



According to Greef,* parthenogenesis appear to occur in 

 Uraster ridens. 



In the Ophiioridea, the sexes are distinct,! and the genital 

 organs consist of lobular, pedunculated sacs, which are 

 situated, in pairs, in the inter-radial spaces of the disc. 

 These organs pour their secretions into the peritoneal cavity ; 

 the latter, however, is in communication with the external 

 medium by narrow apertures situated inter-radially on its 

 margins. The ova are fecundated in the water, and in that 



* Murhury Sitzungsherichte, 1 87 1. 



t OjjhiolejJiit sq_aamuta is hermaphrodite. 



