382 li. T. GUNTHER. 



The origin of tlie hood from two lateral ectodermal thicken- 

 ings on either side of the mouth has already been described. 



The fate of the mesoderm and of its paired cavities is of 

 the highest importance. In Sagitta, after a transient stage, 

 in which the body cavities are entirely obliterated, two bilate- 

 rally symmetrical pairs of cavities appear as schizocoels, the 

 head cavities and the gonadial cavities respectively. The 

 chief function of the mesoblast of the anterior or head pair of 

 cavities is apparently to give rise to the musculature of the 

 pharyngeal apparatus. The cavities themselves during the 

 process of growth become extended into the base of the hood 

 thus proving this organ not to be purely ectodermal in origin, 

 a fact which is of considerable importance from the point of 

 view of homology with the interbrachial membrane of the 

 Cephalopoda. 



The fate of the perigonadial coelomic cavities is very similar 

 in Chtetognatha and Mollusca, and, in our opinion, the 

 ChEetognath type of ccelom, gonnds and their ducts represents 

 the ancestral Molluscan condition with greater exactitude 

 than any scheme hitherto proposed. 



In spite of the great variations in the arrangement of the 

 Molluscan coelom it is generally agreed that the most primi- 

 tive condition is that in which the gonadial and reno-pericardial 

 cavities are united as in the Cephalopoda and in the Aplaco- 

 phoran Amphineura, as in a modified degree in the more 

 archaic Gastropoda (Trochus) and Lamellibranchia (Soleno- 

 mya), and as is indicated by the origin of the gonads from the 

 wall of the pericardial space in Paludiiia and Dreissensia. 



This view has been expressed by Dr. Pelseneer in a series 

 of diagrams indicating the probable transformations of the 

 genital duct (Mollnsca, fig. 5 bis, p. 14), but in delineating 

 the "ancestral hypothetical form" Dr. Pelseneer has not 

 carried the history as far back as is now possible, for he has 

 omitted to emphasise the real cause of the bilateral symmetry 

 of the parts, namely, the primitive independence of right and 

 left coelomic cavities, necessitating the development of inde- 

 pendent right and left ducts whether genital or renal. 



