56 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLLSCA. 
system is by no means a test of high organization in the Mollusca, but rather the 
reverse. 
A peculiarity of the Mollushs belonging to the second and third categories, viz. 
that their infraoesophageal nervous mass is often perforated by the aorta, may be 
accounted for by the narrowness of the angeial ring, consequent upon the concentra- 
tion of the elements of the parieto-splanchnic system, so that they unite directly above 
the aorta. 
The singular variation in the arrangement of the different portions of the nervous 
system, whereby the Mollusca as a class differ so widely from the other great classes 
of Vertebrata and Articulata, may, I think, find an explanation m the well-known law, 
that the constancy of a given arrangement of organs greatly depends upon the period 
at which they appear in embryonic life. If certain organs are formed early, those 
which come later must obviously accommodate themselves to their predecessors ; and 
any variations which have taken place in the latter will perturb the normal disposition 
of the former. 
Now in the Mollusca, as has been already stated, the neural side of the embryo is 
the last to be developed, and the nervous system does not make its appearance until 
the animal has taken its characteristic form. 
Contrast this with the Vertebrata; in them the nervous system is the first to be 
developed, and it is, of consequence, the most fixed and unchanging feature in the 
ivhole of their organization. 
On the other hand, the separation of the abdomen or post-abdomen from the body 
is one of the earliest facts in moiluscoiis development, and it has a corresponding 
influence over their whole organization. 
The Archety-pal Vascular System and its modifications. —It may be questioned 
whether the “archetypal” heart has a single or a double auricle, but it is certain 
that in proportion as the symmetry of the branchial apparatus and of the whole body 
is preserved, we approach to the form of heart with a double auricle. Thus we have 
a double auricle in Chiton and HaUotis, and a close approach to it in Tefhys, Janus, 
and the Eolidce. 
In the Cephalopoda the contraction of the branchio-cardiac veins has been 
observed by Milne-Edwards and Kolliker, so that they may be considered to be 
auricles. This is another curious illustration of the fact, that what is commonly 
considered the most concentrated and highest organization does not occui in the 
reputed highest forms of Mollusca. 
The heart lies above the intestine, and gives off the aorta anteriorly. Ihis 
runs forwards through the cephalo-pedal ring with the oesophagus, and terminates 
eventually in the buccal mass. Its main branches may be classed as visceral and 
pedal. 
It is needless to enter here upon the beautiful discoveries of M. Milne-Edwards, 
with respect to the incompleteness of the circulation in the Mollusca. Ihe facts I 
