50 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
From all that has been stated, I think that it is now possible to form a notion of 
the archetype of the Cephalous IVIollusca, and I beg' it to be undei stood that in using 
this term, 1 make no reference to any real or imaginary ‘'ideas” upon which animal 
forms are modelled. All that I mean is the conception of a form embodying the most 
general propositions that can be affirmed respecting the Cephalous jNIollusca, stand- 
ing in the same relation to them as the diagram to a geometrical theorem, and like 
it at once imaginary and true. 
The archetype of the Cephalous Mollusca, then, it may be said, has a bilaterally 
symmetrical head and body. The latter possesses on its neural surface a peculiar 
locomotive appendage, the foot ; which consists of three portions from before back- 
wards, viz. the propodium, the mesopodiiim and the metapodium, and bears upon 
its lateral surface a peculiar expansion, the epipodium (Plate V. fig. 1). 
The heemal surface of the archetype may or may not secrete a shell upon its surface, 
or in its interior. 
If we compare this unmodified form with the vertebrate or articulate archetype, 
we find that the three essentially correspond. The appendicular system of the Verte- 
brata and Articulata is represented by the epipodium in the Cephalous Mollusca 
(Plate V. figs. 9, 10, 11). 
Nevertheless the differences between the three archetypes are so sharp and marked, 
as to allow of no real transition between them. 
In the Cephalous Mollusca it is the heemal side of the hody which is first developed. 
In the Articulata and Vertebrata it is the neural side which first mahes its appear- 
ance. 
The archetype of the Cephalous Mollusca further differs from that of the Vertebrata 
(and resembles that of the Articulata), in the circumstance, that while in the latter 
the nervous and intestinal axes are parallel, in the former they decussate; that is, the 
oesophagus opens on the neural side, passing between the great nervous com- 
missures. 
The molluscous archetype again differs from that of both Vertebrata and Articulata 
in its appendicular system (cjo), which, when it exists, never presents articulations nor 
anything that can be called an external or internal skeleton (unless indeed the funnel- 
cartilages of Cephalopoda be such), and which is generally altogether suppressed m 
the adult state, its place being supplied by the foot, which, as a development of the 
central neural region into a locomotive organ, is, so far as I am aware, paralleled 
throughout the Vertebrata and Articulata by nothing but the dorsal fin of a fish. 
In the actual forms, the symmetry of the archetype is almost always disturbed by 
in which the shell is wound is the same as that in which the intestine is bent. While the aperture of the 
shell therefore is “ihsemad” in Atlanta with regard to the axis of columella, it is “neurad” in Nautilus and 
Argonauta. 
With an internal shell the reverse appears to be the law. Hence the curvature of the shell of Spirula is the 
opposite of that of the shell of Nautilus, and that of a pulmonated Gasteropod is the same as that of a Pectini- 
branch. 
