46 MR. T. H, HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
velopment of the Nudibranchiata, Tectibranchiata and Pectinibranchiaia, tends to 
the conclusion, that in them the visceral mass is thrust out hehind the anus ; is in 
fact a 'post-ahdomen*. Plate V. figs. 6-8. 
A little consideration will show that the intestine drawn into this must become 
bent towards the hoemal side, as in fact it is in the embryos of all three groupsf. _ 
Upon erabryological grounds, then, we should establish two great primary modifi- 
cations of the molluscous archetype ; the one characterized by the development of an 
abdomen, and a consequent neural flexure of the intestine ; the other marked by the 
development of a post-abdomen, and the consequent hcemal flexure of the intestine. 
But these modifications of anatomical structure exactly correspond with those 
which I have already demonstrated, upon anatomical grounds, to occur in the Ptero- 
poda and Heteropoda; and I trust I am not overstepping the bounds of legitimate 
analogy in assuming that the anatomical fact of a neural flexure indicates the em- 
bryological development of an abdomen ; that of a hsemal flexure, the development 
of a post-abdomen ; and that therefore the Pteropoda fall into the same category with. 
the Cephalopoda and Pulmonata ; the Heteropoda into that of the Pectimbranchiata, 
Tectibranchiata and Nudibranchiata, 
It is remarkable, that, as regards the flexure of the intestine, similar contrasted 
modifications of the archetype take place in those animals which are the nearest allies 
of the Mollusca; I mean the Ascidians and Polyzoa, the Molluscoides of Milne- 
Ed WARDS. In each of these groups the intestine is always bent upon itself; but while 
in the Ascidian the bend is always hoemal, in the Polyzoon it is neural. The lattei 
fact is evident to any one who will examine a Polyzoon ; the former may seem at first 
sight to be contradicted by the circumstance, that the ganglion in the Ascidians is 
placed between the cloacal and branchial apertures. However, as I have endeavoured 
to show elsewhere:^;, whatever be the position of the anus in the Ascidians, the first 
bend of the intestine is always haemal. I have already referred to their probable 
analogy with Sph'ialis in this respect. 
Plaving now determined the changes which take place in the axis of symmetry of 
the Mollusca, let us examine into those undergone by their principal external organs. 
Whether we have to do with a Cephalopod or with an ordinary Mollusk, the first 
step in development is the separation of the blastoderm into a central elevation, the 
* See particularly Letdig, Ueier Pahdina vivipara, Siebold and Kolliker’s ZeitscLrift, 1850, u'here the 
thru«tinff forwards of the anus by the development of the mantle is particularly shown, p. 142. 
“ A considerable change in the position of the anus takes place when the fold of the mantle becomes fomed 
and moves forward, because thereby the intestine and anus are also thrust forward, and to the right side. 
t The development of the Pectinibranchiata cannot be said to have been carefully worked out yet, with the 
exception of that of Pahdina, but what has been done tends to the conclusions above stated. 
+ Upon the Anatomy and Physiology of Salpa and Pyrosoma, Philosophical Transactions, 1851, and in a 
“Report upon the Structure of the Ascidians,” read before the British Association, September 1852. The law 
as regards the Polyzoa was first enunciated by Professor Allman, “ On the Homology of the Organs of the 
Tunicata and Polyzoa,” Trans. Royal Irish Acad. vol. xxii. 
