120 GENERAL CONSIDERATIONS. 



In the Mollusca the coelom and vascular space have not 

 been generally sufficiently distinguished from one another. 

 There seems, however, to be no doubt that the pericardial 

 cavity of the Lamellibranchiata and Gasteropoda repre- 

 sents the entire coelom. The reasons for this conclusion are 

 (1) the pericardial cavity is always shut oflp from the vascular 

 system ; (2) it communicates with the exterior by a pair of 

 nephridia. 



The generative organs have no relation to the coelom, so 

 far as is known, in either of the above Molluscan gronps ; but 

 in the Cephalopoda the generative cells are developed from 

 the mesoderm lining a certain part of the coelom. This gene- 

 rative part of the coelom seems, however, to be shut oflF in 

 the adult from the viscero-pericardial sac. 



This fact, viz. the coelomic nature of the generative organs 

 of the Cephalopoda, together with the fact that in other 

 Molluscs the generative organs either dehisce into one of the 

 nephridia, which morphologically are part of the coelom, or 

 possess ducts which open close to or into the nephridial ducts, 

 seems, to say the least of it, in favour of the view that the 

 generative organs of all Molluscs were originally coelomic and 

 that the present arrangement found in the majority is secondary. 

 The question, of course, can only be settled definitely by em- 

 bryological investigations, but, unfortunately, embryology does 

 not speak clearly on the point. 



There can then, from the point of view of adult anatomy, be 

 but little doubt that the pericardial cavity (and viscero-peri- 

 cardial and generative sacs in Cephalopoda) alone is coelomic 

 in the Mollusca, and that the other system of spaces whether 

 simulating a body cavity as in Chiton and other Gastero- 

 poda, or forming a close meshwork of spaces as in Lamel- 

 libranchs, are vascular and non-ccelomic spaces; and it is 

 only necessary for embryology to bear out this conclusion to 

 settle the matter definitely. Unfortunately, embryologists have 

 not for the most part sufficiently regarded in their investigations 

 the importance of the point, and, for the majority of Mollusca, 

 we are in ignorance as to the exact method of development 



