244 CHITONID. 
The above remarks perhaps furnish us with the proper value 
of the arguments of malacologists in favour of the Chitons 
being, by the peculiar disposition of the testaceous covering, 
allied to the Articulata. 
It is scarcely necessary to observe, that the usual single 
powerful muscle of attachment of the animal to the shell im 
the Patelloid tribe, is in Chiton, from a necessity arising from 
the disunited structure of the cone, converted into a minuter 
series of coordinate muscles to attach it and each section of 
the shell in its proper position. 
It will now be convenient to look at the anatomy of these 
animals, of which the most important feature are the medul- 
lary masses, and for an account of them we refer to the 
descriptive notes on Chiton fascicularis, in which will be seen 
the unmistakeable cesophageal collar of the Mollusca, without 
a trace of the longitudinal knotted or ganghonic cordon of 
the Annelida, or any of that division of the Articulata termed 
Crustacea, including the Cirripoda. This poimt alone is, 
perhaps, decisive of the question at issue. The next consi- 
deration are the organs of the circulation; these, by being 
disposed on a mesial line, dorsally, and more externally than 
in the strict Gasteropoda, have a greater alliance with the 
conchiferous type; and as they exhibit some unusual varia- 
‘tions in their composition, it will be necessary to examine 
with detail the extraordiary, I believe unique, structure of 
these organs, and I hope to arrive at such probable con- 
clusions as will account for the rationale of this abnormal 
disposition. 
The posterior position of the motive power of the circulation 
will, I think, assist in solving this problem, and must always 
be kept in view. 
If we divide the longitudinal area of the animal into eight 
equal sections, the heart will be found near the hinder extre- 
mity, and may be considered as composed of two inflations 
connected by an intermediate marked strangulation; but 
between them, there is an isochronal systole and diastole 
action; the anterior inflation is of an elongated oval shape, 
and the largest; the posterior is considerably smaller and 
