SEA-FISHERIES LABORATORY. 425 
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 
