ON THE DEVELOPMENT OFCUCUMAUIA. 631 



30. The Early Developiiient oE Cacumaria : Preliminary 

 Account. By H. G. Newth, A.R.C.S., F.Z.S., 

 Demonstrator of Zoology at the Imperial College of 

 Science and Technology. 



[Received May 3X, 1916 ; Road October 24, 1916.] 

 (Plates I., II., and Text-figure 1.) 



Index. Page 



Introduction 631 



Material and Method 632 



The Living Young 633 



Formation and Segmentation of the Coelom 634 



The Pentacula of C saxicola 636 



References to Literature 639 



Exjjlanation of the Plates 640 



Introduction. 



The study of the development of the various groups of 

 Echiiioderms has attracted many workers during recent years, 

 and the result of their labours is a remarkably complete and, 

 on the whole, remarkably consistent chapter of embryological 

 research. Only in the case of the Holothurioidea has there 

 been left any considerable hiatus in our knowledge of the normal 

 development of a group. Uiidoubtedly many causes have con- 

 tributed to this defect, but chiefly it is due, I think, to the 

 difficulty of artificially fertilizing the eggs and raising cultures 

 of the larva3 in the laboratory. The forms hitherto investigated 

 have accordingly been investigated incompletely, or they have 

 been viviparous, as in the case of Synapta vivipara, or they have 

 been such as present a " shortened " larval life and can therefore 

 be readily reared, as in the case of Cticumaria. 



The main outlines of the development of the Auricularia are 

 described in the works of Semon (13) and Bury (1 and 2), 

 who both investigated the same species — Synapta digitata ; and 

 Clark (3) has given an admirable account of the embryology of 

 S. vivipara. It has been urged, however, with much reason, that 

 the synaptas are unsuitable forms to study, on account of the 

 departure of their adult anatomy from the typical Holothurian 

 plan. " The fact is that Synapta is about the worst form that 

 could have been chosen to represent the Holothurioidea. Its 

 radial water- vascular canals are only transitory larval structures, 

 and its buccal tentacles not only spring directly from the water- 

 vascular ring, but, in contravention of the rule which prevails in 

 all the other groups of Holothurioidea, their number is no longer 

 a multiple of five " (MacBride, 8). We owe to Ludwig the only 

 fairly complete account of an ontogeny to which the above 

 objection docs not apply. His description of the development of 



