MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 63 
logical relations does the branchial sac of an Ascidian correspond with the mantle- 
cavity of a Lamellibranch. 
The nervous system is totally different. The three pairs of ganglia, which exist in 
all Lamellibranchiata (even the apodal genera), are replaced by one in the Ascidians, 
which is not homologous (as is commonly asserted) with the branchial ganglion, or 
intersiphonic ganglion of Lamellibranchiata, but with their pedal ganglion. 
The organization of the circulatory system is wholly different. The Ascidians 
have a cellulose test, not a calcareous shell. The larval conditions are totally distinct. 
If, however, all Cephalous Mollusks, i. e. all Cephalopoda, Gasteropoda and Lamelli- 
branchiata, be only modifications by excess or defect of the parts of a definite arche- 
type, then, I think, it follows as a necessary consequence, that no anamorphism takes 
place in this group. There is no progression from a lower to a higher type, but 
merely a more or less complete evolution of one type. 
It may indeed be a matter of very grave consideration whether true anamorphosis 
ever occurs in the whole animal kingdom. If it do, then the doctrine that every 
natural group is organized after a definite archetype, a doctrine which seems to me 
as important for zoology as the theory of definite proportions for chemistry, must be 
given up. 
Description of the Plates. 
In all the Plates the same letters refer to the same parts. 
a. 
Anus. 
0. 
Ovary. 
b. 
Buccal mass. 
P- 
Penis. 
c. 
Contractile sac. 
pp. 
Propodium. 
d. 
Subspiral ciliated band. 
9- 
Abdomen. 
ep. 
Epipodium. 
r. 
Post-abdomen. 
/ 
Salivary gland. 
s. 
Vesicula seminalis. 
or 
C' • 
Tentacles. 
t. 
Testis. 
h. 
Head. 
u. 
Auricle. 
i. 
Eye or optic nerve. 
'd. 
Venous canal. 
j- 
Auditory vesicle or nerve. 
V. 
Ventricle. 
k. 
Stomach. 
w. 
Aorta. 
k’. 
Pyloric caecum. 
w'. 
Recurrent artery. 
1 . 
Liver. 
X. 
Cerebral ganglia. 
m. 
Mantle. 
y- 
Pedal ganglia. 
mt. 
Metapodium. 
y- 
Pedal artery. 
ms. 
Mesopodium. 
Parieto-splanchnic ganglia. 
n. 
Branchiae. 
groups have one definite archetype or primitive form upon which they are all modelled ; another, to imagine 
that there exist any transitional forms between them. 
Every one knows that Birds and Fishes are modifications of the one vertebrate archetype ; no one believes 
that there are any transitional forms between Birds and Fishes. 
K 2 
