12 raOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



the conflicting statements must be wrong ; the probable explanation 

 being that we have here one of those details in which diiferent species 

 vary from one another. 



(b) The pedal commissures. — Here again, as regards the anterior pedal 

 connnissure, my observations nphold Von Jhering to the disadvantage 

 of Haller : for the latter,' in contradiction to Yon Jhering's- assertion 

 that the anterior commissure considerably exceeds its successors in 

 size, denies that it can be distinguished from the rest in any way. 

 In Hanleya ahijssorum the anterior pedal commissiire is a simple 

 unbranched strand passing directly from one pedal cord to the other 

 (Fig. 11, f.c.'). In size it is about equal to the suboesophageal com- 

 missure, and is strikingly larger than its companions, for whereas 

 they could only with difficulty be seen at their orij:in from the pedal 

 nerves, the anterior commissure could be easily traced and removed.^ 

 The comparative difference in size between the anterior pedal com- 

 missure and the roots of its successors can be seen in Fig. 11, j^.c, pj. 



As we follow the pedal cords to their posterior extremity, we find 

 that they gradually approach each other, till at the extreme end of 

 the foot they appear to become continuous, forming a loop comparable 

 to that made by the lateral cords above the rectum. In reality, 

 however, this is not exactly the case, for the examination of a series 

 of sections through this region makes it evident that a short tract 

 uniting the two cords is devoid of ganglion cells, so that they must be 

 said to be united by a short, stout, fibrous commissure (Fig. Ill, c). 



Fig. III. — Posterior extremity of the pedal nerve-cords of Hanleya ahjssormn. x 30 

 (reconstructed from sections), c. fibrous commissure. «. nerves di])pin(c 

 into musclts of tlie foot (these nerves are about equal in size to the 

 preceding pedal commissures), p.n. pedal nerve-cords. 



{c) The heart. — The heart, in certain parts of its structure, seems to be 

 a most interestingly variable organ in the different species of Chiton. 

 From a paper published last year by Haller,* one can recognize no less 

 than four different types dependent on the number and position of tlu' 

 auriculo-ventricular openings. At the bottom of the scale he descriljcs 

 four pairs of such openings in Chiton magnijicus ; then comes the con- 

 dition found in most Chitons, two pairs ; after that, one pair with 



' Loc. cif. pt. 1, p. 12. 



- Von .Jhering, " Verglcichende Anatomic des Nervensystems uud rhylogeuie der 

 Molluskeu," p. 4.5. 



•' No. 1305 C, Physiological Series, Roy. Coll. Surg. Museum. 

 * Haller, Morph.'Jabrb., xxi, 1894, p' 29. 



