1/8 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



produces a corresponding neural flexure of the intestine, as in Cephalo- 

 poda, Pteropoda, and Pulmonata ; that of a post-abdomen produces a 

 li(£inal flexure, as in Heteropoda, Pectinibranchiata, Tectibranchiata, 

 and Nudibranchiata (Plate V. [Plate 20] flgs. 6, 7, 8). 



From combinations of these primary changes with subsequent 

 greater or less developments of the various parts of the foot, all the 

 varieties of form in the Cephalous Mollusca are produced. ^ 



The formation of an abdomen with a peculiar development of the 

 margins of the foot into elongated processes, and with cohesion of the 

 posterior epipodial lobes, gives us the Cephalopodan subtype. 



The formation of an abdomen with an excessive development of 

 the epipodium, at the expense of the foot-proper, characterizes the 

 Pteropoda. 



The formation of an abdomen with a moderate development of 

 the foot-proper, and hardly any of the epipodium, marks the Pul- 

 monate subtype. 



The Heteropoda combine a great development of the foot-proper 

 \\\\\\ the formation of a post-abdomen (and only a transitory develop- 

 ment of the epipodium ?). The Pectinibranchiata seem to differ from 

 them only in degree. 



The development of a post-abdomen, coexistent with that of the 

 epipodium, characterises the Tectibranchiata. 



The Nudibranchiata have a post-abdomen and an epipodium in 

 their embryonic condition, but lose both (epipodium ?) more or less 

 completely as they attain maturity. The foot-proper is very mode- 

 rately developed, or even disappears i^Phyllirrhoe). 



If the "mantle" is to have an analogue anywhere among the 

 Articulata or Vertebrata, it ma)' probably be with the carapace of the 

 Crustacea, inasmuch as this is developed from a corresponding region 

 and has similar functions, i.e., to protect the respiratory organs. 



Hitherto what has been said has referred to the morphology of the 

 external organs. It remains to show on what plan the internal organs 

 are arranged, and how the archetypal arrangement is modified among 

 the different families. To enter upon this subject fully would belong 

 rather to a formal treatise upon the Mollusca. For the present my 

 object is merely to point out the fundamental unity which obtains 

 among certain of the most important systems of organs, and to bring 



^ For clearness' sake I have referred to the "htemal" and "neural" flexures as if they 

 always took place in a vertical plane, whereas, as every one knows, the anal aperture is 

 almost always either to the right or the left in the Gasteropoda. The only modification of 

 the theory required to meet this fact, is to suppose that the hfemal outgrovvth takes place 

 more rapidly on one side than on the other. 



