92 



pigment granules are tlien seen to be present to a small 

 extent in the cells, they are not frequent enough to give 

 any conspicuous colour, and the organs diti'er greatly 

 from the very obvious pigmented osphradia of many 

 molluscs. Two small ganglia, the osphradial ganglia, 

 have already been described in the section on the nervous 

 system. They lie in close proximity to the visceral 

 ganglion, and nerves pass upwards from the ganglion cells 

 forming these two spherical ganglia to the surface 

 epithelium, almost directly above, where there is a small 

 prominent area of elongated epithelium forming the 

 osphradium. If a piece of the adductor muscle is 

 removed so that it bears on its surface the visceral ganglia 

 and overlying connective tissue and epithelium, and 

 sections are cut at right angles to the surface, the 

 osphradial ganglia and their connections with the 

 osphradia can be easily followed. The epithelial cells of 

 the osphradium increase in height until they are about 

 three times that of the adjoining ordinary epithelium. 

 The cells forming the organ bear no cilia, though these 

 occur on the ordinary adjacent epithelial cells, but there 

 is a prominent cuticle present. The nucleus is large and 

 almost round, and small pigment granules occur (though 

 in small numbers) in the cytoplasm. Underlying the 

 osphradia are nerve cells connected with the innervating 

 nerves from the ganglia, and from these, numerous fibrils 

 arise and pass through the connective tissue basal 

 membrane, until, branching still further, they pass 

 between the supporting epithelial cells to the surface 

 (text-fig. 3). 



The Abdominal Sense Organ was described first by 

 Thiele in a species of Area in 1887 (39), and List in 

 1902 (6) described the structure in detail in the Mytilidae. 

 In Pecten maxinms it is well developed but is unpaired. 



