Anatomy of Cuttlefishes. 95 



animal in appearance though not in structure than is 

 the eye of any other group of invertebrates 



Cephalopods were abundant in former ages, but 

 there are now not more than 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 sticker-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 dis- 

 tinctive characters of the group and its divisions. 

 They are all soft-bodied, never distinctly divided 

 into segments nor provided with 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 right and left, heart composed of auricle 

 and ventricle, gills lamellar = Class I. Lamelli- 

 branchiata or Acephala. 



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



period of existence) = Class* 5 !!. 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. 



