98 



The microscopic structure is almost identical with that 

 in Mytilus, and probably this organ occurs generally in 

 the lamellibranchia. It is a slight uncoloured thickening 

 situated on a connective tissue " flap " which passes from 

 the adductor to the right mantle lobe, just above the last 

 point of attachment of the rectum to the muscle. The 

 free edge of this thin flap is directed towards the hinge 

 line. 



Fig. 3. Section through Osphradium. x 450. 



If transverse sections are made through the ridge, 

 the striking microscopic structure of this organ makes it 

 at once obvious. It appears (see fig. 4) as a hillock of 

 sense cells extending on both sides of the edge of the 

 connective tissue flap, and much longer than it is thick. 

 It is rendered very evident because firstly the cells 

 composing it are many times higher than the ordinary 

 epithelium and the junction is very distinct, and secondly 

 because the sense hillock bears a thick covering of 

 extremely long cilia or fibrils, which are themselves 

 several times the length of ordinary epithelial cells and 

 occur nowhere else on the body. The sensory epithelium 

 rests on a basal connective tissue membrane which is 

 pierced by nerves, or rather by nerve fibrils, which 

 proceed from the visceral ganglion and pass directly over 

 the adductor under its connective tissue sheath. In 

 transverse sections stained with haematein the epithelium 



