XI 



THE SHELLS OF PTEROPODS 



833 



have no biological grounds for such a statement ; for the shell arises 

 from a single embryonic origin, and afterwards becomes spHt into 

 portions which constitute the two separate valves. \¥e can perhaps 

 throw some indirect light upon this phenomenon, and upon several 

 other phenomena connected with shell-growth, by a consideration 

 of the simple conical or tubular shells of the Pteropods. The shells 

 of the latter are in few cases suitable for simple mathematical 

 investigation, but nevertheless they are of very considerable interest 

 in connection with our general problem. The morphology of the 

 Pteropods is bv no means well understood, and in speaking of them 



B 



Fig. 410. Diagrammatic transverse sections, or 



outlines of the mouth, in certain Pteropod shells: 



Fig. 409. Pteropod shells: ^' ^' ^^^^^''^ australis; C, C. pyramidalis; 



(1) Cuvierimi columnella; D, CMUintium; E, C. cuspidata. After Boas. 



(2) Cleodora chierchiae; 



(3) C. pygmaea. After Boas. 



I will assume that there are still grounds for beheving (in spite of 

 Boas' and Pelseneer's arguments) that they are directly related to, 

 or may at least be directly compared with, the Cephalopoda*. 



The simplest shells among the Pteropods have the form of a tube, 

 more or less cylindrical {Cuvierinay, more often conical (Creseis, 

 Clio) ; and this tubular shell (as we have already had occasion to 

 remark, on p. 416), frequently tends, when it is very small and 

 delicate, to assume the character of an unduloid. (In such a case 

 it is more than likely that the tiny shell, or that portion of it which 



* We need not assume a close relationship, nor indeed any more than such a 

 one as permits us to compare the shell of a Nautilus with that of a Gastropod. 



