cl Introduction. 



as also towards their distal ends. Organs of special sense, in the 

 shape of otolithic vesicles, have been observed in Synaptidae ap- 

 pended to the radial nerve-cords just where they pass through the 

 foramina in the radial ossicles of the internal calcareous ring. 



With the exception of the Bynaptidae, the Holothurians are 

 always dioecious, resembling in this all other Echinodermata, but 

 differing from them in never having the radiate arrangement re- 

 tained in their generative glands. These organs take the shape of 

 longer or shorter, simple or branched coeca, which are attached to 

 one or both sides of the inter-radial dorsal mesentery, and discharge 

 their products by a single duct running in the same medio-dorsal 

 line to open either posteriorly to, or within the circle of oral tentacles. 

 In one case, HolotJmria tremula, the development has been observed 

 to be nearly direct, the provisional organs being represented merely 

 by a rapidly-disappearing layer of ciliated sarcode ; but in most cases 

 _ there are two larval or pseudembryonic forms, the earlier of which, 

 Auricularia, possesses a mouth and alimentary canal, and special 

 bilaterally symmetrical natatory lobes, whilst the second, the 

 so-called ^pupa,' is barrel-shaped, devoid of mouth and of lateral 

 outgrowths, and girded with five zonular ciliated bands, which dis- 

 appear as the ambulacral system is developed. 



Class, Eehinoidea. 

 Echinodermata varying in shape from that of a sphere to that of 

 a disc, with, in all living species but one, in which the skeleton has 

 been reported to be flexible, an immovably articulated external 

 shell, the so-called ^corona,' which, with its ambulacral and inter- 

 ambulacral spaces, occupies the entire exterior of the body, with the 

 exception of a small circum-oral and a small anti-ambulacral cir- 

 cum-anal area. When the mouth and anus are at opposite poles, 

 the Eehinoidea are called ^regular,' or ^ endocyclica ;' when the anus 

 is placed excentrically, they are called ^irregular,' or 'exocyclica.' 

 The two types are mutually connected by transitional forms ; the 

 irregular shows bilateral symmetry very obviously; the bivium, 

 however, and the trivium do not hold the same relations to the 

 dorsal and ventral surfaces as in Holothurioidea. The locomotor 

 feet are in many of the regular forms uniformly sucker-shaped, and 

 provided with a distal calcareous support; but the dorsal feet, even 

 m some regular forms, may lose their sucker-shape ; and in the 



