Anatomy of Cuttlefishes. 95 



animal than is the eye of any other group of inverte- 

 brates. 



Cephalopods were abundant in former ages, but 

 there are now about 230 species living; of these the 

 Nautili possess a chambered shell, four gills, and 

 many tentacles, while all others have only two gills, 

 and eight or ten sucker-bearing arms. 



Recapitulation. Having thus very briefly re- 

 viewed the sub-kingdom Mollusca, we may, by way of 

 recapitulation, place in a tabular form the distinctive 

 characters of the group and its divisions. They are 

 all soft-bodied, never distinctly divided into segments 

 nor provided with jointed limbs ; enveloped more or 

 less in a dermal mantle, which often secretes a shell, 

 and their larval stage is usually ciliated, worm-like. 



The divisions are : 



A. Having no distinct head, bivalve shells with the 



valves dorsal and ventral ; no separate gills = 

 Class I. Brachiopoda. 



B. Having no distinct head, bivalve shells with the 



valves right and left, gills lamellar = Class II. 

 Lamellibranchiata or Acephala. 



C. Having a distinct head, univalve shells (at some 



period of existence) = Class III. Cephalo- 

 phora. 



a. Entirely enclosed in a mantle, secreting a 



tubular shell = Sub-class I. Scapho- 

 poda (Dentalium). 



b. Not entirely enclosed in a mantle, swim- 



ming by finlike processes on the upper 

 side of the foot = Sub-class II. 

 Pteropoda. 



