MORPHOLOGICAL DIFFERENTIATION. 329 



anterior part of the foot being differentiated into special 

 grasping organs auxiliary to the mouth functions. 



The specialization of the tubular mode of locomotion and 

 the differentiation of the foot into a funnel and tentacles are 

 characteristics of this highest type of mollusk ; and its relation 

 to the Pteropod wings is seen in the fact that in the Dibran- 

 chiate order the lateral lobes are fused together to form a 

 closed tube — the siphon, while in the Tetrabranchiate order 

 they are only brought close together, and not fused into a 

 continuous tube. There is also the differentiation of distinct 

 swimming flappers in some of the Dibranchiates, in addition 

 to the siphon, which is specialized as an organ for distribution 

 of ink into the water, and b)^ darkening the water compen- 

 sates for its slow rate of escape by locomotion from any cause 

 of danger. 



The Structure of the Cephalopods. — Although the purpose 

 of this volume does not include the detailed description of 

 organisms, a better understanding of the remarks that follow 

 may be reached by a brief review of the essential structural 

 elements of the Cephalopods. For this purpose the following 

 translation of extracts from Zittel's description will be useful, 

 and for further details the reader is referred to his excellent 

 Handbook of Paleontology.* 



First Order, Tetrabranchiata. — Cephalopods with shell; furnished 

 with four branching gills, or branchiae, funnel formed by union of two 

 lobes of foot, but not permanently united; no ink sac or pouch. In the 

 place of arms, numerous tentacles, slender, elongated and without suck- 

 ers or hooks; shell chambered. 



The Animal. — All that we know of the organization of the Tetrabranchi- 

 ata is based upon the genus Nautilus, the only one of the order now liv- 

 ing, the shell of which is seen in most museums; but the animal is very 

 rare, and has been seen alive in only a few instances. The animal occu- 

 pies the last chamber of the shell, with the ventral side turned outward 

 (the coiling of the shell thus being toward the dorsal side); the body is 

 short and thick; the head separated from the trunk by a slight constric- 

 tion. In place of arms, about ninety contractile filiform tentacles inserted 

 in muscular sheaths surround the mouth; they are grouped in sev- 

 eral bundles, and in an order a little different in the male and female. 

 The tentacles situated on the dorsal side are soldered together to 

 form a thick muscular lobe which can close the opening of the shell 

 when the animal has withdrawn into the last chamber. The funnel is a 



* " Handbuch der Palaeontologie, i. Abllieilung: Palaozoologie," von Karl 

 A. Zittel, vol. II, 18S1-1885, pp. 332, etc. 



