THE CHJITOGNATHA, OR PRIMITIVE MOLLUSC A. 375 



Leioglossal Cephalopoda, and that it even tends to disappear 

 in certain groups of carnivorous Grastropoda. 



A comparison between the forms of teeth characteristic of 

 Ch^tognatha and of Amphineura is instructive (text-fig. 8). 



Compare the saw-like hooks of Krohnia ha mat a and S. 

 serrato-dentata {a, b) with a tooth of Lepidomenia {d), 

 and the teeth of S. bipunctata (c) with those of Proneo- 

 menia (e). 



No special salivary glands have been noticed, but the 

 alimentary canal of Sagitta cephaloptera is furnished 

 with two (liver?) diverticula (text-fig. 1, a, h.). 



Genital Organs. 



The genital organs of the Chastognatha are situated in the 

 hinder part of the visceral sac. The gonads are paired and 

 hermaphrodite, but the male and female gonads develop in 

 separate cavities, and pass to the exterior by separate paired 

 ducts. The Mollusca too are typically hermaphi-odite, and 

 although there are some reasons for regarding the dioecious 

 condition as primitive in the group, both ova and sperma- 

 tozoa are derived from the epithelial lining of the same 

 cavity ; but in Mollusca belonging to very different groups 

 there is a tendency for the male and female gonads to 

 develop apart, either in separate male and female acini 

 (Pleurobranchidge, Nudibranchs, Cardium obi on gum, 

 etc.), or in separate regions of the same gland (Cycladidae), 

 or in different glands (Auatinacea and Septibranchia). The 

 longitudinal partition between the paired gouadial cavities 

 of the Chgetognatha reminds us of the division between the 

 paired gonads of the Aplacophora, and is typical, there is 

 good reason to believe, of the primitive condition in Mollusca 

 (text-fig. 10). 



Thanks to the minute observations of Doncaster we are 

 able to draw attention to an astonishing similarity between 

 the behaviour of the gonads both of the Mollusca and of the 

 Chsetognatha, which we believe may throw a new light upon 

 the morphology of the transverse septum of the latter group. 



