EVOLUTION 127 



fewer and more powerful teeth than Nautilus. The 

 latter has four teeth on either side of the median, 

 with basal plates representing two others, whilst the 

 rest have three on either side, sometimes flanked out- 

 side with a single basal plate, while in one genus 

 only, Gonatus, but two laterals are left on either side. 



The circulatory system can be adduced as showing 

 development if those of the tribe at the head of the 

 phylum (the Dibranchiate Cephalopods) and of the 

 primitive Gastropods be contrasted ; for in the latter 

 the circulatory system, instead of branching off into 

 capillaries, is distended into swollen, irregular cavities 

 and sinuses, which are, so to speak, insinuated among 

 the various organs of the body, while a certain 

 amount of the blood finds its way back to the heart 

 without passing through the respiratory organs. The 

 Dibranchia, on the other hand, have the most com- 

 plete circulatory system of any mollusc, the blood 

 being nearly entirely contained in true vessels. 



The molluscan heart, at the same time, offers 

 some anomalies when the different groups are com- 

 pared. It is most primitive, and more nearly 

 approximates the annelidean type, in Nautilus, where 

 the single ventrical (and no mollusc has more than 

 one) is served by four auricles, whereas in all the 

 other symmetrical Mollusca it has but two auricles 

 (except in the Scaphopoda and Aplacophora, in 

 which the heart is rudimentary). 



In the streptoneurous Gastropoda, proportionately 



