THE ANATOMY OF PLEUROTOMARIA BEYBICHII. 227 



more mediau position being only an apparent one due to tlie 

 displacement of the mantle in the dissection. However this 

 may be, I have utterly failed to find any such structure, 

 either internally or externally to the branchial ganglion ; and 

 if I am correct in my identification of the osphradium I 

 cannot help doubting the osphradial nature of the structure 

 to which Dall assigns this significance in P. Adansoniana. 



While examining a series of transverse sections through 

 the free end of the gill I noticed a thickened patch of epithe- 

 lium situated on the ventral or external border of the gill, 

 and extending from near the point of attachment to the free 

 end (figs. 5 — 7, os.). An examination of these sections 

 shows that this thickened patch of epithelium overlies the 

 great branchial nerve, and receives numerous branches from 

 it. The epithelium itself has all the character of a sensory 

 one, consisting as it does of delicate fusiform cells supported 

 by more columnar ones, aud also exhibiting a few pigment 

 cells. Although a few gland cells are to be seen they are 

 much less numerous here than in the adjacent epithelium. 

 At times there is even a suggestion of the bunching together 

 of these cells into oval masses, not unlike the taste bulbs of 

 the Vertebrata and the sensory organs ofAcavus Walton i, 

 as described by the Sarasius. 



The identity of this strip of scnsoiy epithelium with the 

 osphradium is confirtned by a comparison with the latter 

 organ, as seen in Haliotis. In this genus the osphradium, 

 as described by Spengel, has precisely the same relationship 

 and form as the strip of seusory epithelium found in Pleuro- 

 tomaria Beyrichii, which last we may, I think, safely 

 identify as the osphradium. 



At the posterior end of each gill the afferent and effer- 

 ent branchial vessels may be seen (figs. 5 and 7, a. h. and 

 e. b.). The former, springing from a sinus (figs. 7 and 23, 

 V. s.) situated ventrally to the rectum and ureter, run forward 

 near the former structure and diverge outward to the gills; 

 while the latter, passing on either side of the mantle-cavity, 

 converge on the heart. As we have already seen, the afferent 



