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, i is distended into swollen, irregular cavities 
the various organs of the hoe. while a Scere 
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 
pared. “It is is most OSE eee 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 
