MR. T. H. HUXLEY ON THE MORPHOLOGY OP THE CEPHALOUS MOLLUSCA. 51 
the excessive development of a peculiar region of the haemal surface, into what has 
been termed the abdomen or post-abdomen, according as it is placed before or behind 
the anus. 
The integument of this outgrowth, commonly modified in structure and having 
frequently a prolonged anterior or posterior margin, is the ‘^mantle.” It may or 
may not secrete a shell. 
The development of an abdomen (Plate V. figs. 2, 3, 4, 5) produces a correspond- 
ing neural flexure of the intestine, as in Cephalopoda, Pteropoda and Pulmonata ; that 
of a post-abdomen produces a hcemal flexure, as in Heteropoda, Pectinibranchiata, 
Tectibranchiata and Nudibranchiata (Plate V. figs. 6, 7, 8). 
From combinations of these primary changes with subsequent greater or less deve- 
lopments of the various parts of the foot, all the varieties of form in the Cephaloiis 
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 Pulmonate subtype. 
The Heteropoda combine a great development of the foot-proper with the form- 
ation of a post-abdomen (and only a transitory development of the epipodium r). 
The Pectinibranchiata seem to differ from them only in degree. 
The development of a post-abdomen, coexistent with that of the epipodium, 
characterizes 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 moderately developed, or even disappears {Phyllirrhoe) . 
If the “ mantle” is to have an analogue anywhere among the Articulata or Verte- 
brata, it may probably be with the carapace of the Crustacea, inasmuch as this is 
developed from a corresponding region and has similar functions, ^. 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 
* For clearness’ sake I have referred to the "haemal” 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 
haemal outgrowth takes place more rapidly on one side than on the other. 
