THE OSPHRADIA 195 



with the object, apparently, of testing the quality of the water 

 before it passes over the branchiae. It consists of a patch of the 

 epithelium, modified in a special manner, and connected by its 

 own nerve with one of the visceral ganglia. 



An osphradium does not necessarily^ occur in all genera ; for 

 instance, it has not been detected in Fissurella. It is most 

 highly specialised in the Conidae, and in the carnivorous Gas- 

 teropoda generally. In Buccinum undatum^ for instance, it is 

 very large indeed, and, from its plumed form, has sometimes 

 been mistaken for an accessory branchia (Fig. 95). In Haliotis 

 it is paired, one lying in close proximity to each of the two 

 branchiae, but in Turbo it 



is single, corresponding to ^^ ^r___^^J^ 



the single branchia. In ^^^^^'^ ^^^^^^^~-~-^ 



Chiton there is an osphra- . ^^^f \ ^^ix^^^^k: '^^ 



dium at the base of each ^"-y-^^^^.m^-^^^^^^^s^^'.^'^^^W^ 

 separate gill filament, mak- .^^^^^^^^^wl^^wif^ 

 ing a total of twenty or [ yyVfiJ vu^^^^^^^^P^ 



more on each side. Its '""^"^'^^-■^^^^'^^^^^^^^ 



position in Physa and in Fig. 95. — ^Mccmwm wnda^wm L., deprived of 



Cydostoma will be seen by i^' '\')^^,'^^°'^lf ^ T /'^^*!'^^ p''''^'°'' ""* 



^ . "^ branchia {hr) and osphradium (os) ; m, mucous 



reference to Figs. 103 and glands ; s, siphon. The portion of the mantle 

 104 Td 205^ In the Pele- covering the osphradium has been removed. 



cypoda the osphradia are paired, and lie adjacent to the pos- 

 terior adductor muscle, close to the hinder end of the axis of 

 the branchiae. In the Tetrabranchiate Cephalopoda there are 

 two osphradia, placed between the bases of the two pairs of 

 gills. In the Dibranchiates on the other hand, a groove above 

 the eyes has been regarded as the seat of the organ of smell. 

 This groove contains sensory and ciliated cells, and appears to 

 be connected with a special nerve centre of its own, which ulti- 

 mately is derived from the cerebral ganglion. 



Scarcely any instances of the exercise of the sense of smell 

 on the part of bivalve Mollusca have been recorded. Some- 

 thing of the sort, however, seems to have been present in a case 

 related by Mr. R. L. King.i A skull of a fox had been placed 

 in a small ditch in order to soak, and after a few days, when 

 taken out, was found to be covered with Pisidium pusillum to 

 the number of at least two hundred, which had been probably 

 1 Zoologist, iv. p. 1266. 



