2^*^ s. GOTO; 



is well developed, but in a star, such as is represented in Fig. 16, in 

 which the ambulacral feet have been developed I have not been ;ible to 

 detect any muscle fibres in the wall of the water- vascular system, 

 although it can adhere to the substratum quite hard. The muscula- 

 ture of the hydrocœl is, like that of the enterocœl, developed on the 

 external surface of its epithelium. 



On the origin of the genital organs I have absolutely no observa- 

 tion to offer. In individuals whose radius Avas 5 mm. I was not able 

 to find any trace of them. 



IV Cavities of Wlesenchymatous Origin 



In this category I reckon the periha^mal spaces, i.e., with the 

 exception of the inner perih^vmal ring-space which I have proved to 

 be a true enterocœl, and the peribranchial cavities. 



l'erihœiiial siistem. — My observations on the origin of this 

 system have given a very different result from that of MacBride ['96]. 

 It originates in a star which has just completed metamorphosis, as 

 solid masses of mesenchymatous cells on the oral side of the radial 

 water-vascular canals. They are situated in pairs on either side of 

 an imaginary plane passing through the axis of each radial canal and 

 cutting the disc at a right angle. They are also invariably present 

 between the bases of the ambulacral feet, and the pair nearest the 

 centre of the disc is situated at first between the water-vascular ring- 

 canal and the first pair of ambulacral feet. Subsequently, however, 

 it comes to lie directly on the oral side of the former. Tiiese solid 

 masses of cells afterwards acquire lumen, and in this stage they are 

 represented in Fig. 28. The cavities are at this stage entirely separate 

 from one another, but they afterwards gr(3w and fuse with each other, 

 with this limitation that the jnesentery lying between each pair 

 persists, so that the spaces on one side of the radial plane com- 



