THE OSPIIRADIA 



195 



Ff( 



object, apparently, of testing the quality of the water before it 

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

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

 own nerve with one of the visceral ganglia. 



An ospliradium does not necessarily occur in all genera; foi- 

 instance, it has nut been detected in Fissu.rrlla. It is most 

 highly specialised in tlie Couidae, and in the carnivorous 

 Gasteropoda generally. In Buccinum undatnm, for instance, it is 

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

 been mistaken for an accessory l)ranchia (Fig. 95). In Huliofi^ 

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

 Ijranchiae, but in Turho it 

 is single, corresponding to 

 the single branchia. In 

 Chiton there is an osplira- 

 dium at the base of each 

 separate gill filament, mak- 

 ing a total of twenty or 

 more on each side. Its 

 position in Physa and in 

 Gydostoma will be seen by 

 reference to Figs. 103 and 

 104 (p. 205). In the Tele- 

 cypoda the osphradia are paired, and lie adjacent to the posterior 

 adductor muscle, close to the hinder end of the axis of the branchiae. 

 In the Tetrabranchiate Cephalopoda there are two osphradia, placed 

 lietween the bases of the t^vo pairs of gills. In tlie Diltranchiates 

 on the other hand, a groove aljove the eyes has been regarded as 

 the seat of tlie organ of smell. This groove contains sensory -and 

 ciliated cells, and appears t(j l)e c(jiinected with a special nerve 

 centre of its own, which ultimately 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. Something of 

 the sort, however, seems to have lieen present in a case related by 

 Mr. E. L. King.-' A skull of ;i fox bad been ])laced in a small 

 ditcli in order to soak, and after a i'ew days, when taken out, was 

 found to l)e covered with Pisidhim pusillum to the number of 

 at least two hundred, which bail Ijcen probably attracted from the 

 ' Zoolv'jlsl, iv. p. 1-266. 



. 95. — Buccinum undatum L. , dejirived of 

 its shell, showing the relative position of 

 branchia (br) and osphradiuiu (o.s) ; m, mucous 

 glands ; s, .si2ihon. The portion of the mantle 

 covering the ospliradium lias been removed. 



