SITUS INVERSUS IN EOHINOIDS 131 



work on Ophiothrix fragilis (13, pp. 578, 586) that 

 this sac, homologous with the left hydrocoele, exhibits varying 

 degrees of development among normal larvae, and in a few 

 extreme cases it gives rise to a five-lobed hydrocoele (PL xxxvi, 

 fig. 54 ; compare further those double-hydrocoele Ophioplutei 

 described by Mil Her and Metschnikof f). 



Whether this unusual development of the right hydrocoele 

 is to be regarded as a case of atavism or as another kind of 

 variation is a matter of choice. MacBride (14, pp. 240, 244) 

 is of opinion that the free-swimming ancestor of the Echino- 

 derm had a pair of hydrocoeles, equally developed on each 

 side, the right one has, however, become atrophied as soon as 

 the free-swimming habit was given up. The appearance in 

 some abnormal larvae of a right hydrocoele is an atavistic 

 feature. But, according to him, the appearance and further 

 completion of the associated structures, such as amniotic 

 invagination, set of spines and dental sacs, derived from the 

 ectoderm and mesoderm respectively cannot be accounted for 

 by atavism, because it is quite impossible to endow the 

 ancestor with such a double set of highly-developed spines and 

 Aristotle's lanterns. Therefore, he introduced the idea of the 

 internal secretion, in that the abnormally-developed right 

 hydrocoele must have given off some stimulating substances 

 which caused both ectoderm and a part of the posterior coelom 

 to respond, with the result that there appeared a second 

 set of spines and dental sacs. He further discussed this 

 theory in his second paper on the double hydrocoele (15, 

 pp. 341-5). Some months earlier than the first of these papers 

 Grave (9, p. 43) discussed the same idea and made the objec- 

 tion ' that such an explanation presupposes that the series of 

 structures in question was present and in some way related 

 in the normal development of the ancestral echinoderm, 

 a supposition for which there is no basis in observed fact '. 



Now, we may find no great difficulty in assuming that such 

 stimulating power of the left hydrocoele has been acquired 

 since the disappearance of the right hydrocoele, as v. Ubisch 

 (30, p. 444) remarked in reply to Grave's objection. It 



