Mr. W. Clark on the Chitonidse. 275 



apparatus of the circulation have a rectilinear dorsal arrangement, 

 similar to that of the Annelida : Cuvier and Lamarck regard 

 them as true Mollusca, ranging with the Patelloid group : Pro- 

 fessor Forbes has doubts, and looks on the question as still 

 within the limits of debateable ground, and terms the Chitons 

 malacological " puzzles." Some observers contend, that the re- 

 productive organs, unlike the asymmetrical ones of the Gastero- 

 poda, exhibit a disposition of parities on a medial line, and like 

 M. De Blainville refer them to the Annelida. Milne-Edwards 

 demurs that they are Mollusca, and goes no further than to 

 regard them as an aberrant tribe of Gasteropoda. 



Having dissected many examples of three species, I think that 

 my notes may assist zoologists in coming to sound conclusions 

 with respect to natural position. As my investigations have in- 

 duced a chain of reasoning which has convinced me that the 

 Chitons are true Mollusca of the patelloid type, it may be as well 

 at once to allude to that part of them which bears upon the 

 objections that have just been stated. 



Though doubts have lately sprung up as to the natural position 

 of these curious animals, they have, until now, been placed by 

 most authors in close connection with the Conchifera. If this is 

 right, what then is there extraordinary and unusual in the dis- 

 position of the organs of the circulation ? They have nearly the 

 same dorsal rectilinear position as in the Acephala, from which 

 they have long been considered, and I think it will be shown 

 rightly, the point of transition to the Gasteropoda. Why not, 

 therefore, contrast this peculiar arrangement which is the inva- 

 riable consequence of the symmetry of the bivalve cone, with 

 that which obtains in the Chitons from the same cause, and also 

 in others of the Patelloid tribe that have the same position and 

 a similar parity of their organs ? I admit, that the strict P«^e//^, 

 though symmetrical in their testaceous cones, are exceptions with 

 regard to the heart, auricle, and branchial plume : Haliotis, which 

 with me is a patelloid animal, and also an exception, is the reverse, 

 having the organs of the respiratory circulation symmetrical, but 

 not the cone. These, and two or three other genera, may be 

 regarded as the precursors of the Gasteropoda, and points of 

 transition from the strict parities of the cone of the shell and 

 organs of the Patelloida, to the asymmetrical division of the 

 Gasteropoda. 



The only differences, and they are not important, with respect 

 to the position of the circulation in the bivalves and Chitons are, 

 that in the latter the motive power is placed greatly more pos- 

 teriorly than in the former; and the illustrious Cuvier has taught 

 us to observe, that the auricles of the Chitons have a quadruple 

 connection with the heart, of which he has seen no other example 



