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11. On the Morphology of the Cephalous Mollusca, as illustrated hy the Anatomy 
of certain Heteropoda and Pteropoda collected during the Voyage of H.M.S. 
“Rattlesnake" in 1846—50. 
-Bi/ Thomas Henry Huxley, Esq., F.R.S., Assistant Surgeon R.N. 
Received March 18, — Read May 13 and 27, 1852. 
“ XhE mere description of appearances even of the interior structure, still less of 
the exterior surfoce of an animal, without the deductions which they legitimately yield, 
is of comparatively small value to the philosophic naturalist ; for of what value are 
facts until they have been made subservient to establishing general conclusions and 
laws of correlation, by which the judgment may be safely guided in regard to future 
glimpses at new phenomena in nature*?” 
If I prefix this admirable exposition of the true aims of anatomical investigation 
to the present essay, it is that I may justify, by the highest authority, the course which 
I have taken in considering what of new facts it contains, as of subordinate import- 
ance to the reasonings which may be based upon those facts ; in making its scope 
more morphological than zoological. 
The morphology of the Cephalous Mollusca is a subject which has been greatly 
neglected. No Savigny has determined the homologies of their different organs, and 
so furnished the only scientific basis for anatomical and zoological nomenclature. 
It is not settled whether the back of a cuttle-fish answers to the dorsal or to the 
ventral surface of a Gasteropod. It is not decided whether the arms and funnel of 
the one have or have not their homologues in the other. The dorsal integument of a 
Doris and the cloak of a Whelk are both called “mantle,” without any evidence to 
show that they are really homologous. 
Nor do very much more definite notions seem to have prevailed with regard to 
the archetypal molluscotis form, and the mode in which (if such an archetype exist) 
it becomes modified in the different secondary types. So far as our knowledge goes 
among other forms of animal life, we invariably find that, whatever the subsequent 
variations and aberrations, the primordial embryonic form has its parts arranged 
symmetrically about a given axis. 
No one imagines the Pleuronectidee belong to an asymmetrical type because they are 
asymmetrical in their adult shape, and yet there is no stronger evidence for the very 
common assertion that the typical form of the Mollusca is spiral or asymmetrical'f'. 
This unsatisfactory state of our knowledge appears to me to result from two causes ; 
Owen, Anatomy of Spirv.la, Voyage of Samarang, Zoology, p. 12. 
f See Von Baer, Nova Acta Acad. Nat. Cur. vol. xviii. p. 753. 
MDCCCLIII. 
F 
