THE MOLLUSCS OF THE GRRAT AFRICAN LAKES. 



189 



(a) Nassopsis nassa. 



During life this mollusc inhabits the surface rocks of Tan- 

 ganyika, and its shells are always richly encrusted with the 

 green algae which clothe the rocks for a considerable depth. 

 It is sluggish, and appears to browse within a very limited 

 area, like the Patellas of the ocean beach. The foot is broad, 

 somewhat pigmented, and quite white in places; the snout is 

 broad, black, and wrinkled, not protrusible, but retractile. 

 The tentacles are short and black, and the eyes are not carried 



- - int. 



Crist.S.S. 

 a.crists.s. 



-b.ci. 



S.C. 



Fig. I. — Stomach of Nassopsis uassa opened from above, showing relation 

 of cr^'stalline style-?ac to main chamber of stomach, b i. Bristle passed 

 from aperture of oesophagus along its interior, h z. Bristle passed from 

 aperture of intestine to the cut end of same. Crist, s. s. Crystalline 

 stjle-sac a. crist. s. s. Opening between stomach and crystalline 

 style-sac. l.d. Aperture of digestive gland. S. c. Spiral caecum. 



