MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 47 
mantle, and certain lateral portions. Now these portions become in the Gasteropoda, 
the head and foot ; in the Cephalopoda, the head and arms. It follows therefore that 
the arms of the Cephalopod are homologous with the foot of the Gasteropod. 
Again, in the Cephalopod an eminence becomes developed along two lines, which 
run on each side of the upper part of the ‘‘ lateral expansions” and meet behind the 
head ; along the anterior portion of this line the eminence remains as a slight ridge, 
which afterwards becomes one of the muscles of the funnel ; along the posterior 
portion of the line a considerable process is developed, and, uniting with its fellow, 
becomes the funnel. 
In the Gasteropod, it is along the anterior halves of two corresponding lines that 
processes are developed, which become the ciliated alee or vela of the embryo. The 
line in question I propose to call the epiyodial line, and whatever is developed along, 
that line I consider to be the epipodium, or a portion of it. I do not venture upon such 
a refinement at present, but I think it probable, that as we have distinguished three 
portions in the foot, so it will be necessary to distinguish three portions in the epipo- 
dium ; anterior, middle and posterior. For instance, in the Cephalopoda the posterior 
portion only is developed as the funnel ; in the Gasteropod larvae the ciliated vela 
are the homologues of its anterior portion. The palrnated lobes of the Turbinidm, 
the “lobes of the mantle” of Aplysia, appear to be developed from the whole epipo- 
dial line, while it is apparently the middle epipodium alone which is developed into 
the “wings” of the Pteropoda. 
All traces of the epipodium appear to have vanished in the majority of thePectini- 
branchiata*. 
Of all mollusks Atlanta possesses the best developed ybo^-proper, and has its parts 
* Leuckart and Loven have enunciated very different views with regard to the homologies of the external 
organs of the Mollusca, to which it seems proper I should refer. 
Leuckart, for instance {op. cit. pp. 155-59), considers that the anterior cephalic lobes of the embryo Cepha- 
lopod answ'er to the cephalic velum of Gasteropoda ; the posterior cephalic lobes to the alse of Pteropoda, 
while the funnel corresponds with the middle lobe of the foot. The arms he considers to be peculiar struc- 
tures, mere appendages to the cephalic lobes. 
If the halves of the funnel, however, answer to the middle lobes of the foot, how is it that they unite upon 
the dorsal surface of the neck ? If the anterior cephalic lobes answer to the vela of Gasteropoda, how is it that 
the latter disappear, and do not contribute to the formation of the head in Gasteropoda ? Finally, it must be 
remembered that the arms of the Cephalapoda arise quite independently of the cephalic lobes, the first deve- 
loped arms being those most distant from the head. 
Leuckart considers that the oral lobes of the pulmonate embryo are the homologues of the ciliated vela of 
Gasteropoda. But their position and number are against this view. It seems to me that these oral lobes cor- 
respond with the cephahc lobes of the embryo Cephalopod, and it has been well shown by GEGENBArrR {op. cit.) 
that the whole so-caUed “yelk-sac” of the Pulmonata is the true homologue of the vela in Pectinibranchiata ; 
the “ciliated bands” of Van Beneden and Windischmann turn out to beWolffian bodies, and to be internal, 
not external organs. 
The common view, that the alae of the Pteropoda are the persistent vela of the embryo, is, so far as I am 
aware, unsupported by any evidence. Embryology teaches us hitherto that the anterior part of the epipodium 
H 2 
