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



Grube, has been most carefully described by Job. Muller, 1 Selenka, 2 Danielssen and 

 Koren, 8 and Kovalewsky. 4 Accorcbng to these authors the gastrula, as it grows larger, 

 assumes a distinctly bilateral form ; the ventral surface becomes more or less flattened 

 or concave, the dorsal, on the contrary, convex, and the latter projects slightly beyond 

 the mouth and terminates anteriorly, as in Cucumaria doliolum, in a rounded prse-oral 

 prominence (Kopfkegel, according to Selenka). The mouth is thus fully ventral. As 

 development advances, the larva loses more or less of its primitive bilaterality, and the 

 mouth becomes more terminal in position. In fact, a bilateral symmetry is distinctly 

 traceable even in many adult forms of shallow-water Holothurids, as, for instance, Psoitis, 

 Colochirus, and the Aspidochirotse, but, as it seems to me, this bilaterality is nowhere so 

 conspicuous as in the deep-water Holothurids in question. In many Elasipoda the convex 

 dorsal surface projects further than the mouth, which is thus rendered thoroughly ventral 

 in position. That portion of the back which lies in front of the mouth strikingly resemble 

 the rounded prae-oral prominence of the larva of Cucumaria doliolum. 



As a matter of fact, the first pedicels which become developed not only belong to 

 the ventral surface, but are even disposed in pairs and situated near the posterior 

 extremity of the larva. It is rather surprising to find numerous examples of deep-sea 

 Holothurioidea, as, for instance, Elpidia purpurea, Scotopjlanes robusta, most of the 

 species of Pcniagone, Scotoanassa, &c., which are provided with only a few pairs of pedicels, 

 situated on the posterior part of the ventral surface, while the rest of that surface is com- 

 pletely devoid of pedicels. Moreover, the pedicels of the Elasipoda belong exclusively to 

 the ventral surface, and are distinctly opposed across that surface so as to form pairs with 

 each other. Thus it seems as if even with respect to the pedicels, the conformity 

 between the larvae and the adult in the Elasipoda is more striking than that which 

 exists in the Apoda and the Pedata. 



The simplest forms of calcareous deposits are spicules, and these also appear first in the 

 body-wall of the larvae of tbe Pedata, while it is a well known fact that the larvae of the 

 Synapta are provided with small wheel-shaped plates, which are evidently a much more 

 complicated kind of deposit than the former. The perisoma of the Elasipoda, excepting 

 Deima and Oneirophanta, is regularly strengthened by spicules and wheels, the former of 

 which are far more common than the latter. Thus it must, I think, be admitted that the 

 Elasipoda present a singular resemblance to the larval forms as to their calcareous deposits, 

 these having remained at such a low degree of development. As a matter of fact, simple 

 unbranched spicules alone are found in several species, but spicules with three or four arms 



1 AbhaiuUungen d. Ktinigl. Akad. d. Wissenschaften zu Berlin, 1846-1854. 



2 Zur Entwickelung der Holotliurien (Holothuria tubulosa and Cucumaria doliolum), (Zeitschrift fur wissen- 

 schaftliche Zoologie, xxvii., 1S76, pp. 155-179, pis. ix.-xiii.). 



3 Bidrag til Holothuriernes udviklingshistorie (Fauna littoralis Norvegise, ii., 1856. pp. 47-54, pis. vii., viii.). 



4 Entwickelungsgeschichte der Holotliurien (Memoires de lAcad. imp. d. Sc. de St. Petersbourg, vii. serie, 

 torn, xi., No. 6, 1867, pp. 1-8, pi. i.). 



