182 AFFINITIES AND GEOGRAPHICAL 



families of the preceding j for " they all," says a minute ob- 

 server of nature, " commence life under the same form, both 

 of shell and animal ; namely, a very simple spiral, helicoid 

 shell, and an animal furnished with two ciliated wings or lobes, 

 by which it can swim freely through the fluid in which it is 

 contained. At this stage of the animal's existence, it corre- 

 sponds to the permanent state of a Pteropod." 1 



In the univalve mollusks, as in the bivalves, it clearly ap- 

 pears that the humblest families are destined to a fixed place 

 in the depths of the ocean. As we advance through the higher 

 groups, we find in parallel steps with an improvement in the 

 organs of animal life, an advance in the sphere of existence to 

 a life on the surface of the ocean, to fresh water, and even to 

 dry land. The humble Helicidcn (snails), a family of the Gas- 

 teropoda, are the first animals which we encounter as adven- 

 turing upon the firm surface of the globe. And it is interest- 

 ing to remark, in this progression, the requisite change in the 

 mode of respiration namely, from branchiae, the apparatus 

 necessary in aquatic life, to a vascular air-sac, the first form of 

 lungs the proper breathing organ of terrestrial animals. 



In the peculiarly destructive Cephalopoda, we recognise the 

 highest organization of which the molluscan form appears 

 capable ; it includes the orthoceratites, ammonites, belemnites, 

 etc. of the rock systems, and the nautilus and cuttle-fish of the 

 present era. Their descent is probably from the carnivorous 

 families of the pteropoda ; for " the nucleus of their shells," says 

 the naturalist last quoted, " is a spiral univalve, similar in form 

 to the undeveloped shells above alluded to [those of the em- 

 bryo gasteropods] ; and it is yet to be seen whether all cepha- 

 lopoda do not commence their existence under a spiral-shelled 

 pteropodous form." It has also been remarked, that "the 

 shells of two species [pteropoda] afford indications of a transi- 

 tion towards the cephalopoda ; one resembling in its straight 

 conical form the belemnite and many other extinct genera of 

 that class, and the other having a partially-formed chamber at 

 the lower closed extremity ; and similar evidence is afforded 

 by their internal structure.'" 2 This genealogy, if it shall be 

 affirmed, will afford an important illustration of the geological 

 history, because it will show that cephalopoda might le expected 

 to make their appearance as early in the rock series as any other 

 mollusks possessing parts equally fitted to commemorate their ex- 



1 Professor Edw. Forbes, in Jameson's Journal, xxxvi. 326. 

 2 Carpenter's General Physiology. 



