BtTIlNE : ANATOMY OF HANLFA'A AEYSSOKUM, !) 



cells, for such, on comparison with Bernard's ' figiurcs, I take certain 

 dark staining streaks lying between the supporting cells to be, occur 

 chiefly in this strip of elevated epithelium, but are also scattered 

 throughout the low epithelium covering the rest of the walls of the 

 blood-vessels. 



The locality, pigmentation, and minute structure of this strip of 

 elevated epithelium, when added to the ganglionic character of the 

 extei'nal branchial nerve of Ilanleija ulnj>isoruiii, appear to warrant us 

 in regarding this region of the gill as the seat of a sense organ, in 

 all probability the representative of the osphradium in more higldy 

 specialized molluscs. The fact that the hair cells are not strictly 

 confined to this modified portion of the epithelium suggests a certain 

 diffuse sensibility, a condition in accord with the well-known lack of 

 centralization in archaic creatures. 



Althougli the general structure of this sense organ corresponds to 

 that of an osphradium, yet before we conclude that such is indeed 

 its nature, there are two considerable objections that must be 

 mentioned. In the first place it is related to the wrong branchial 

 vessel ; in all cases the osphradium is in connection with the aiferent 

 branchial vessel, whereas this organ is on the etierent. With 

 reference to this difficulty, it may be worth noting that a sense organ 

 for testing the purity of the water would iirobably be so situated 

 as to be most easily and rapidly accessible to the stream of water 

 coming to the gills, a condition realized more nearly on the outside 

 than on the inside of the gill ; but Avhether such a change of position 

 in a very constant organ can depend on so slight a physiological 

 advantage may very well be doubted. The second difiiculty occurs 

 in the relations borne by the lateral nerve-cord to the viscera. Among 

 the higher molluscs an osphradium invariably receives its nerve supply 

 from the visceral loop, a nervous loop passing from one pleiiral ganglion 

 to the other, situated morphologically entirely beneath the intestine. 

 The lateral nerve-cord of Chiton, however, which from its relations to 

 this assumed osphradium would be homologous to the visceral loop, 

 passes above, not beneath the viscera. 



This latter difiiculty can, of course, be met by a supposed migration 

 of the anus during the course of evolution ; but perhaps a less violent 

 escape is afforded by Hubrecht's^ ingenious suggestion that possibly 

 the posterior pedal commissure lost its original connection with the 

 pedal cords, but remained united to the lateral by a pair of stout 

 latero-pedal connectives ; at the same time the normal posterior union 

 of the lateral cords degenerated and vanished, thus transforming a 

 dorsal into a ventral loop. Either of these alternatives is possible, 

 but the magnitude of the assumptions necessary woiild excuse a 

 certain amount of scepticism with regard to their probability. 



If, in spite of these two difficulties — difficulties, I may add, which 



' F. Leruard, " Siir les Org-ancs Palleaux dos rrosobninclios " : Aun. Sci. iiat. 

 IS'JO. 



- A. A. W. Ilubrecht, "■' Prutieonieitia Slni/eri" : Niedei'l. Arch. Zool. IbSl, p. 2-5. 



