EXTERNAL FEATURES. 



ANATOMY OF THE MOLLUSCA. 



EXTERNAL FEATURES. 



The Cephalopoda are immediately distinguished from all 

 other classes of the mollusca by the circle of acetahula or 

 arms (xxiii), Tihich surrounds the head; these were formerly 

 regarded as homologous with the foot of the gastropods, with 

 its margins produced into the eight or ten processes which are 

 indifferently designated as arms, acetahula or brachia, but the 

 siphon (xxiii, 9), is more largely used as an organ of locomotion 

 than are the arms, and its ventral position, as well as the 

 distribution of the nerves, indicate that it, rather than the arms, 

 is the homologue of the foot of the gastropod. , 



The mouth is supplied with a pair of calcareous or horny 

 jaws, resembling in form the beaks of a parrot (ix, 93, 95). The 

 arms proper, in the naked cephalopods, are eight in number, and 

 are distinctively called sessile arms, to distinguish them from 

 the tentacular arms or tentacles, much longer, and capable of 

 retraction within pouches, which are additionally possessed by 

 those species which have an internal shell or cuttle-bone. The 

 sessile arms are provided with suckers (xxiii, 4), or hooks for 

 prehension, covering their internal surface, whilst the tentacles 

 are expanded into club-shaped terminations similarly armed 

 (xxv, 20). In the Nautilus, sole living representative of an 

 exceedingly numerous extinct order, the arms degenerate into 

 numerous tentacles (iv, 62, 63), unarmed, which are retractile 

 into eight sheaths, morphologically representing the typical 

 eight arms. In this animal only, the body is contained within, 

 and protected by, an external shell, which compensates to it the 

 loss of the offensive and locomotive powers possessed by its 

 more highly organized allies. 



Behind the arms, the circle of which may constitute its crown, 

 that major portion of the head is situated which is usually 

 distinctively so designated ; its dorsal aspect exhibits on either 

 side an eye. The head may be joined to the body by a more or 

 less constricted neck, or is frequentlj^ sessile upon the latter, 

 without intervening constriction. The body, or mantle, is 

 dorsally more or less continuous with the head, but ventrally 

 forms a sack anteriorly open, and from which emerges the 

 funnel or siphon. The sack or body is, in a small portion of the 

 octopod and in the decapod species, expanded into postero-lateral 

 membranes, possessing the power of undulatory motion, and 

 which may be considered as the equivalent of fins, in function, 

 though not in appearance. 



The body in the Gastropoda presents the following more or 

 less differentiated portions : 



