XIII] CEPHALOPODA 311 



poda the mantle consists of right and left lobes like those of a 

 Pelecypod and it is by the fusion of these that the tubular form is 

 produced. We should therefore be justified in regarding the Scapho- 

 poda as modified Pelecypoda, were it not for the fact that the 

 Scaphopoda have a well-developed radula sac and complicated 

 muscles for moving it, which constitute a buccal mass like that of 

 the snail. There are likewise buccal ganglia connected with the 

 cerebral ganglia by their own buccal commissure and the cerebral and 

 pleural ganglia are quite distinct. These facts render it impossible to 

 believe that the Scaphopoda are descended from the Pelecypoda and 

 so they must be regarded as an independent division of the 

 Molluscan phylum. 



The Scaphopoda further possess neither labial palps nor ctenidia, 

 and in this also they differ from every Pelecypod known. The mouth 

 is surrounded by a star-shaped frilled membrane and at its sides are 

 inserted two groups of mobile thread-like ciliated tentacles with 

 thickened tips called captacula which Lankester regards as repre- 

 senting the missing ctenidia. How these act in capturing food is not 

 known. The kidneys are two sac-like organs. The genital organ is a 

 median mass of tubes occupying the posterior end of the animal, and 

 it opens into the right kidney. There is no distinct heart but there is 

 a pericardial space and a single longitudinal vessel in the roof of this 

 may perhaps represent the heart. The genus Dentalium is fairly 

 common round our coasts and may be dredged at moderate depths 

 on a muddy bottom as in the Clyde area. 



Class V. CEPHALOPODA. 



The last class of the Mollusca, very differently constituted from 

 the Lamellibranchiata, is that of the Cuttle-fish, or Cephalopoda. 

 This paradoxical name, literally " head-footed " (Gr. Ke^aXif, head ; 

 TTovs, TroSo's, a foot), is suggested by the circumstance that the foot 

 has grown forward and upwards at each side of the head, and that 

 these two extensions have met and coalesced, so to speak, on the 

 back of the neck. The edges of this part of the foot, which may 

 be called the fore-foot, are drawn out into strap-like processes, 

 which are the arms by which the animal seizes its prey. The edges 

 of the hinder part of the foot, on the other hand, have become bent 

 round and joined beneath the animal, so as to form a tube, the 

 funnel, through which water is ejected from the mantle -cavity. 



The best known British Cephalopoda are the Squid, Loligoforbesi, 

 often caught by trawlers, to whom it is known as the 

 " ink-fish " ; Sepia offitinalis, the Cuttle-fish, taken in 



