ON THE DEVELOPMENT OF THE POND-SNAIL. 367 



is a tidrd plane of development really brought about by 

 the formation of a body-cavity, seems to justify the use of 

 the terms Diploblastica and Triploblastica. The latter 

 corresponds essentially with Haeckel's Metazoa. With re- 

 gard to the difference between Professor Haeckel and myself 

 as to the relation of the body-cavity and the water-vascular 

 system, I must at present maintain the view expressed in my 

 essay. The difference is not so great as Professor Haeckel 

 appears to believe. I do not accept his existing groups of 

 acoelomatous worms as such, for in the Planarians and 

 Cestods there appears to me to be evidence that the ramifi- 

 cations of the water-vascular stems are to be regarded as 

 corresponding to a commencing body-cavity. The terminal 

 portions of those stems, which open to the exterior, on the 

 other hand, are, as I pointed out in the essay referred to, to 

 be regarded as something distinct — an involution of the epi- 

 dermic layer subsequently developing into the segmental 

 organ. It does not by any means follow that the body- 

 cavity is primitively open to the exterior, a view which 

 Professor Haeckel has by misapprehension attributed to me. 

 It will not, however, be useful to discuss this matter further 

 without reference to renewed investigation of the facts. 



A second and third phase in the development of the Mol- 

 lusca, which have long been known, and which may or may 

 not make their appearance in any particular case, are (what 

 may be called) the Trochosphere and Veliger forms, the 

 former an early condition of the latter. Both are well known 

 and characteristic of various groups of Worms and Echino- 

 derms, and the latter is seen in its full development in the 

 adult Rotifera and in the larval Gasteropoda and Pteropoda. 

 The identity of the velum of larval Gasteropods with the 

 ciliated discs of Rotifera seems to admit of little doubt, and 

 it would be well to have one term, e.g. velum, by which to 

 describe both. The Trochosphere is the earlier, more or less 

 spherical form in which the velum is represented by an an- 

 nular ciliated ridge, and which is sometimes {e. g. Chiton) 

 provided with a polar tuft of long cilia. 



The cell, polyplast (morula), gastrula, trochosphere, and 

 veliger phases of molluscan development are not distinctive 

 of the molluscan pedigree ; they belong to its prse-molluscan 

 history. The foot, the shell-gland, and the odontophore are 

 organs which are distinctively molluscan — the last character- 

 istic of the higher MoUusca only — the other two of the whole 

 group, and their appearance must be traced to ancestors 

 within the proper stem of the molluscan family tree. The 

 foot is essentially a greatly developed lower lip. 



