278 Mr. W. Clark on the Chitonida. 



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 stracture 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 medullary 

 masses, and for an account of them we refer to the descriptive 

 notes on Chiton fascicularis, in which will be seen the unmis- 

 takeable oesophageal collar of the MoUusca, without a trace of 

 the longitudinal knotted, or ganglionic 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 consideration 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 variations in their composition, it will be 

 necessary to examine with detail the extraordinary, I believe 

 imique, structure of these organs, and we hope to arrive at such 

 probable conclusions 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 heai-t will be found near the hinder extre- 

 mity, and may be considered as composed of two inflations con- 

 nected 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 more subrotund. 

 The auricles receiving blood from the branchial veins commu- 

 nicate in the usual lateral manner with the greater division 

 of the heart, and at this point present their largest calibre; 

 they then form an attenuated arcuation on each side the con- 

 stricted portion, and effect a second contact at the sides of the 

 lesser inflation, which may be regarded as an aortic ventricle re- 

 ceiving the blood by a special auricular apparatus. From the 

 anterior axis of the major part of the heart, a long and large 

 aorta or arterial vein ascends medially to the front, distributing 

 by diverging arteries the aerated fluid to the greater portion of 



