36 



COMPARATIVE ANATOMY 



CHAP. 



on each side of the mantle cavity, one plumose gill, the shaft of which 

 is attached lengthwise to the body (Figs. 45, 46, etc.). 



In various divisions of the Lamellibranchia, the outer organisation 

 deviates very greatly from the above. 



F. Cephalopoda. 



The body is bilaterally symmetrical. The visceral dome is large 



and often much elongated dorso-ventrally. The head is more or less 



distinct, and is surrounded by the foot, 

 <* which is transformed in a peculiar man- 



ner. The foot has, in fact, grown round 

 the head, and has developed numerous 

 differently -shaped processes (arms and 

 tentacles) arranged in a circle round the 

 mouth; these serve principally for seizing 

 and holding prey. In viewing the body 

 of a Cephalopod, it must be remembered 

 that the apex of the visceral dome (which 

 a casual observer might take to be the 

 posterior end of the body) is really the 

 highest dorsal point, while the head and 

 its arms lie lowest. We may thus dis- 

 tinguish, both in the visceral dome and 

 in the transformed foot which has been 

 combined with the head, and drawn out 

 into tentacles, an anterior and a posterior 

 part (which to a casual observer would 

 seem upper and lower parts), and a right 

 and a left side. This at first sight seems 

 a paradox to those not acquainted with 

 the comparative anatomy of the Mollusca, 

 since the normal position in the water of 

 certain well-known Cephalopods does not 

 agree with it. A Sepia, for example, 

 swims or lies at rest in such a way that 

 the strongly pigmented anterior side of 

 the visceral dome and of the " head " 

 (Kopffuss) is uppermost, and the posterior 

 side lowermost. The accompanying dia- 

 gram illustrates the strict morphological 



position of the tody, which alone concerns the comparative anatomist 



(Fig. 47). 



On the right and left of the " head " there is a highly-developed 



eye, and near it an olfactory pit. 



The mantle fold hangs down posteriorly from the visceral dome, 



covering a spacious mantle- or respiratory cavity, which communicates 



FIG. 47. Diagram of Sepia, median 

 section from the left side, v, Ventral 

 (physiologically anterior); d, dorsal 

 (physiologically posterior); an, anterior 

 (physiologically upper) ; po, posterior 

 (physiologically lower) ; 1, 2, 3, 4, 5, the 

 five arms of the left side ; au, eye ; co, 

 internal shell; go, gonad; d, pigment gland 

 = ink-bag ; m, stomach ; n, kidney ; ct, 

 ctenidium ; a, anus ; mh, mantle cavity ; 

 in, siphon. The arrows indicate the 

 direction of the respiratory current. 



