CEPHALOPODA. 



ANATOMY. 



Cephalopod mollusks are i in mediately distinguished from all 

 other classes of the mollusca by the circle of acetabula or arms 

 which surrounds the head ; these may be regarded as homologous 

 with the foot of the gasteropods, with its margins produced into 

 the eight or ten processes which are indifferently designated as 

 arms, acetalmla or brachia. The extension of this modified foot 

 around the mouth of the animal, its manifold uses in its economy 

 and its high structural development, combine to place the 

 cephalopoda at the head of the mollusca.* 



The mouth is supplied with a pair of calcareous or horny jaws, 

 resembling in form the beaks of a parrot. 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 pro- 

 vided with suckers or hooks for prehension, covering their 

 interior surface, whilst the tentacles are expanded into club- 

 shaped terminations similarly armed. In the Nautilus, sole living 

 representative of an exceedingly numerous extinct order, the 

 arms degenerate into numerous tentacles, unarmed, which are 

 retractile into eight sheaths, morphologically representing the 



* The nervous system and phylogeiiy of the cephalopoda have been 

 studied by Hermann von Jhering (Leipzig, 1877), who states as a result 

 of his researches upon the homologies of the ganglia; and nerves, that the 

 arms of the cephalopods in no way represent the foot of the gasteropoda, 

 but are tentacular growths of the head ; that the only part of the cephalo- 

 pod analogous to the foot of the gasteropod, is the siphon. 



