10 TROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



neither prevented Spcngel ' from suggesting, nor Lankcstcr - from 

 accepting, the possible presence of an osphradium upon this identical 

 spot — Ave decide to regard the sense organ in question as the repre- 

 sentative of an osphradium, this vrill be a suitable place to very 

 briefly indicate the influence that an osphradium would possess in 

 regard to one or two questions of Chiton morphology. 



Passing over the interest attached to the mere occurrence of the 

 organ among the Amphincura, the first question with which an 

 osphradium is conceraed is the nature of the gills. Now, although 

 it is usually held that in a Chiton each separate gill is a ctenidium 

 complete in itself, it has been lately maintained that this is not the 

 case,^ but that the gills of a Chiton are processes of the mantle, organs 

 of the lateral line homologous to the sense organs on the epipodium of 

 the Rhipidoglossa and on the mantle of the Lamellibranchs. The 

 presence of an osphradium (if we accept Spengel's view) would at 

 once set this question at rest, for an osphradium is a constant adjunct 

 of a ctenidium. In speaking of this point we have entered an out- 

 lying region of the epipodial controversy, a dispute that naturally 

 suggests another important matter in connection with the osphradium, 

 namely, the nature of the loop formed by the lateral nerve-cords. 

 There are, I fancy, four alternative homologies : it may be considered 

 as homologous to either — (1) the sensory ganglia in the epipodium of 

 llhipidoglossa, the pallial nerve of Lamellibranchs,* etc. ; or (2) the 

 outer division of the pedal nerve-cord of HaliotiH,^ if such a division 

 exists ; or (3) the pleural ganglia and anterior pallial nerves of 

 Anisopleura and Lamellibranchs " ; or, finally, the visceral loop of other 

 molluscs.'' The presence of an osphradium enforces our acceptance of 

 the last alternative, on account of the constant innervation of the 

 ctenidium and its associated osphradium from the visceral loop. 



Such are the reasons, as far as I have been able to ascertain them, 

 that tell both for and against the sensory and osphradial nature of this 

 organ in the above two species of Chiton ; and it must be left to others 

 to judge whether the approximate situation and highly characteristic 

 structure of an osphradium are to be outweighed by certain anomalous 

 details of position ^ and innervation. 



2. Further Notes on thr Anatomy of Hanleya ahyssoi'um. 



(a) The anterior buccal commissure. — The condition of the buccal 

 comraissui'es in this species owes a great part of its interest to the 

 conflicting statements that have been made with regard to this point 



1 Zoe. cit. p. 356. ^ Loc. cit. 



2 Thiele, " Ueber Sinnesorgan . . . von Mollusken," etc.: Zeitschr. "Wiss. Zool., 

 xlix, 1890, p. 411. 



^ Ibid. 



5 H. V. Jhering, Morph. Jahrh., iii, 1877, p. 172. 



^ P. Pelseneer, " Sur repipocliuiu des MoUusques " : Bull. Sci. France et Beige, 

 1891. 



^ Speng'el, loc. cit. p. 3.53. 



^ In this connection the wide separation of the osphradium from its associated gill 

 in Ampullar ia should be noted. 



