62 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
The chambers of the venous appendages, then, in the Cephalopoda answer to a 
“contractile sac,” in which the secreting power and the contractile faculty have 
become restricted and localized in a portion of the organ*. 
I have here touched mainly upon the less commonly understood portions of the 
internal anatomy of the Cephalopoda and Gasteropoda, but they clearly tend to 
strengthen the conclusion to be derived from embryology and the more generally 
known anatomical facts, viz. that the Cephalopoda and Gasteropoda are morphologi- 
cally one, are modifications of the same archetypal molluscous form. 
On the other hand, I have made no reference to the Acephala, nor is it ray inten- 
tion to go into that part of the subject ; but, for the sake of the zoological bearings 
of the question, I may shortly express my belief, that of the two families of the 
Acephala, there is abundant evidence, both anatomical and embryological, to show 
that the one, the Lamellibranchiata'f', is modelled upon the archetype of the Cephalous 
Mollusca. 
Such evidence as we possess with regard to the Brachiopoda, however, is purely 
anatomical, and ('though I am aware that a great weight of authority lies upon the 
other side) yet Mr. Hancock’s opinion, that they are rather to be considered as allied 
to the Polyzoa than to the Cephalous Mollusca, seems to be quite as plausible as the 
more general notion. 
Should this highly ingenious suggestion be found by embryology to be correct, the 
Brachiopoda will have the same relation to the Polyzoa as the simple Ascidians to 
the compound Ascidians, and will form a parallel group to the former in M. Milne- 
Ed wards’s section of “ Molluscmdesr 
In conclusion, I would observe, that the archetypal Cephalous Mollusk (as thus 
defined) is, in all its modifications, sharply separated from other archetypes, what- 
ever apparent resemblances or transitions may exist. In all cases these will, I believe, 
on close examination, be found to be mere cases of analogy, not of affinity. 
As Cuvier long ago remarked of the Cephalopoda and Fishes, so we may say of the 
Cephalous Mollusca in general and other types : — “ Whatever Bonnet and his followers 
may say. Nature here leaves a manifest hiatus among her productions,” For instance, 
great as are the apparent resemblances between a Lamellibranch and an Ascidian, 
they all vanish upon closer examination;!;. Neither in its anatomieal nor in its embryo- 
* A renal organ of similar character has been long since demonstrated in the Lamellibranchiata. (See ^ on 
S iEBOLD, Vergl. Anat.) 
t The Lamellibranchiata are as truly cephalous as many Pteropoda, and the possession of a distinct head is 
so much a question of degree as to be a very unfit classificatory character. 
1 I beg that I may not be misunderstood here. While I consider that there is no transition between the 
Cephalous Mollusca as such, and the Ascidians or Polyzoa, I also fully believe (and so far as the Ascidians are 
concerned I have endeavoured to demonstrate. Report on the Structure of the Ascidians, already referred to) 
that the archetype of the Cephalous Mollusca, that of the Ascidians and that of the Polyzoa, are all referable to 
a common archetype, the archetype of the Mollusca generally. It is one thing to believe that certain natural 
