THE HOLOTHURIOIDEA 225 



disappears, the connection with the body-wall is lost, and the canal 

 opens by a new madreporite into the body-cavity. In some species 

 numerous accessory stone-canals and madreporites may be developed 

 (Fig. II. 5, md). 



The disposition of the organs in the radius, as seen in transverse 

 section, is distinctive (Fig. II. 4). The superficial radial nerve is 

 separated from the epidermis by the thick cutis and a space 

 (epineural canal). The deep radial nerve is separated from the 

 ambulacral radial canal by a pseudhaemal canal and the radial 

 blood-vessel. Over all lies the longitudinal muscle internally. This 

 arrangement resembles most that found in the Echinoidea. 



The alimentary canal and the mesentery which supports it have 

 the dextral coil characteristic of Echinoderma, as described above 

 for Holotlmria (p. 221). 



The respiratory trees, organs quite peculiar to this class, are by 

 no means of universal occurrence, being absent in the Elpidiidae, 

 Pelagothuriidae, and Synaptidae. It is interesting to note, however, 

 that in certain of the Elpidiidae the rectum is provided with a 

 caecum, which may represent a vestige or a rudiment of the respira- 

 tory trees (Fig. III. 10, coe). 



The Cuvierian organs (p. 223) appear to be modified branches 

 of the respiratory trees. 



The Holothurioidea are distinguished from the remainder of the 

 Echinoderma by the structure of the genital organs. These always 

 consist of a single, or of a right and left, tuft of tubules leading into 

 a common duct, which runs in the dorsal mesentery and opens to 

 the exterior in the median dorsal line near the anterior extremity 

 of the body. There is no axial organ or sinus, and no trace of 

 radial structure in connection with the gonads (see p. 24). 



As a rule, the genital products are shed in the sea, where fertilisa- 

 tion and development take place. Rarely, as in Chiridota rotifera 

 (Pourt.) and Pht/llophorus urna (Grube), the young develop in the 

 body-cavity of the parent. Brood chambers are formed in Psolus 

 ephippifer (W. Thomson) and some species of Cucumaria. 



A total and almost equal segmentation of the fertilised egg leads 

 to the formation of a ciliated blastula and gastrula, from which is 

 developed the characteristic Auricularia larva (Fig. I. 6, 7). The 

 free-swimming Auricularia has an alimentary canal provided with a 

 mouth and anus, and the cilia are restricted to a single large 

 circumoral band and a small adoral band within the mouth. The 

 archenteron now gives rise to the hydro-enterocoel, opening by the 

 primary stone- canal at a pore a little to the left of the median 

 dorsal line. Later the circumoral ciliated band becomes greatly 

 folded, and then converted into the circular ciliated rings of the 

 barrel-shaped larva or pupal stage (Fig. I. 8). The mouth shifts 

 to the anterior pole, round which are developed the tentacles as the 



15 



