244 CHITONIDJE. 



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 in 

 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 oesophageal collar of the Mollusca, without 

 a trace of the longitudinal knotted or ganglion ic cordon of 

 the Annelida, or any of that division of the Articulata termed 

 Crustacea, including the Cirripoda. This point 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 extraordinary, 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 



