146 THE VOYAGE OF H.M.S. CHALLENGER. 



of the adult animal. Most of the Elasipoda are remarkable in having the water-vascular 

 system in persistent communication with the exterior, thus obviously resembling the 

 larval state. In the rest of the species, the madreporic canal neither opens externally 

 by a pore, nor does it hang freely into the interior, but its end is intimately joined 

 to the dorsal perisoma and is sometimes, as it were, blind and inserted in it. This 

 must, of course, be considered as a transitional state between the larva and the fully- 

 developed animal. Not long ago Ludwig 1 published an account of a young Chirodota 

 rotifera, Pourtales, in which the madreporic canal had begun to detach itself from the 

 dorsal body- wall ; it had already lost its pore on the exterior, and the blind end was 

 enclosed within the perisoma. 



The respiratory trees and ciliated cups become developed only in a more advanced 

 condition of the larvae. Thus it seems to me as if the persistent absence of such organs 

 in the Elasipoda indicated a certain conformity to the earlier stages of the Holo- 

 thurids. 



It follows from the facts above mentioned that the Elasipoda have retained many 

 peculiarities characteristic of the larvse of the Apoda and Pedata, and consequently that 

 they have in many respects persisted without any sensible change for very long periods 

 of time, and that they do not bear any genetic relation to the present representatives of 

 the Apoda and Pedata, but are derived from ancestral forms of extreme antiquity. 



On comparing the organisation of the recent Holothurioidea — the Elasipoda as well 

 as the Apoda and Pedata — in the different stages of their development, and considering 

 that the development of the embryo records the ancestral history of the species, it seems 

 highly probable that the common progenitors of the three orders of Holothurioidea were 

 characterised by a more or less distinctly-marked bdateral form, by a water-vascular 

 system composed of a circular vessel, tentacular canals and a madreporic canal communi- 

 cating with the exterior, by a calcareous ring, composed of spicules separated from one 

 another, and by the absence of respiratory trees and cibated cups, &c. 



Danielssen and Koren 2 insist on the Elasipoda being placed very low in the series 

 of Holothurids, but in this I cannot quite agree with them. The presence of a well- 

 developed ambulacra! system with five radial ambulacral vessels in connection with pedicels 

 is considered as a marked peculiarity of the Echinoderm type ; besides, in the more typical 

 Echinoderms, as, for instance, the Echinoidea and Asteridea, &c, the madreporic canal 

 terminates beneath a part of the apical system of ossicles, the pores of which place the 

 ambulacral system in communication with the exterior. Now, it seems to me to be 

 rather evident that those Holothurids must be regarded as higher in the Echinoderm 

 chain, in which the water- vascular system has attained a higher degree of development, 



1 Ueber eine lebendiggebarende Synaptide (Archives de Biologie, ii. 1881, pp. 41-56, pi. iii.). 



2 Ecliinodermer fra den Norske Nordhavsexpediticm (Nyt Magaziri for Naturvidenskaberne, xxv. 2, 1879, 

 pp. 102-104). 



