Angular Sea-Cucumher. 147 



remarked that the two classes just mentioned, though they may 

 at fii-st sig-ht, omng to the great differences of their external teg-u- 

 mentary org-anse, appear to be entirely unlike each other, are in 

 reality more closely allied by structural, if not by developmental 

 history, than any other two classes in this sub-kingdom. The 

 Holothurians with rows of ambulacral feet are divided into two 

 families, according to the shape of their ambulacral tentacles ; 

 those in which the tentacles are, as in the Cucumariae, dendritic 

 in appearance, and carried upon a cylindriform stem of different 

 texture from that of the rest of the body, being called ' Dendro- 

 chn-otae ■/ and those in which the tentacles are shield-, or rather 

 shovel-shaped, and in which the integument is continued without 

 any alteration in its texture up to the tentacles, being called 'Aspi- 

 dochirotae/ With these external differences a considerable number 

 of points of difference in their internal structures are correlated, 

 for which see description of next Preparation. In all Holo- 

 thurians, but especially in the otherwise apodal Synaptidae, the 

 ambulacral tentacles are used as locomotor organs; and in the 

 Aspidochirotae they are also used for bringing the sand, in which 

 these animals very ordinarily live partly immersed, into their 

 digestive tract. The intestinal tract, on the other hand, of the 

 Dendrochirotae, in which the tentacles could not be used for this 

 purpose, is ordinarily found to contain no sand or stones. 



For an excellent account of the anatomy of the entire class Holo- 

 thurioidea, see C. Semper's beautifully illustrated Monograph, 

 Reisen im Archipel der Philippinen, Theil ii. ; Wissenschaft- 

 liche Eesiiltate, Bd. i., Hft. i. 1867, pp. 1-6, Hft. iv. 1868, 

 pp. 101-178; Emil Selenka, Zeitschrift fiir Wissenschaftliche 

 Zoologie, Bd. xvii., Hft. 2, 1867. 



For the anatomy and development of the Synaptidae, see a me- 

 moir by Dr. Albert Baur, Nova Acta, vol. xxxi., 1864; where, 

 at p. 50-60, Ab. ii., some valuable remarks will he found upon 

 the ' so-called alternations of generations in the Echinoderms ;' 

 and at p. ly, Ab. i., certain less convincing views as to the 

 homologies of the calcareous ring in the Holothurioidea with 

 the auriculae of Echinoidea. 



e For a note as to the existence of an Echinus with a soft integument, see Semper, 

 I. c, p. 163, citing Giube. 



L 2, 



