202 



larger of the bile ducts (&. d.). The position of this groove and its 

 connection with the oesophagus in Trochus and in Turritella 

 would seem to point to the similarity of the structures in both genera, 

 and it may reasonably be maintained that the small crescentic "ditch" 

 in the stomach of the latter genus represents a vestigial spiral coecum. 



b.d. 



C.(J. 



b.d. - 



int. 



aji. oes. 



ap. int. 



Fig. 2. Fig. 3. 



Fig. 2. Stomach of Trochus linealus ()<^ 4). b. d. apei'ture of the bile duet. 

 c. g. caecal grove, int. intestine, oes. oesophagus, s. c. sjjinal caecum. 



Fig. 3. Stomach of Nassopsis nassa ()x( 3'/.^) (after J. E. S. MoORE). ap. int. 

 opening of the intestine, ap. oes. opening of the oesophagus, b. d. aperture of the bile 

 duct. c. sc. crystalline style-sac. int. intestine, oes. oesophagus, s. c. spinal caecum. 



Such a conclusion seems warranted by the fact that in some 

 primitive forms viz. Nassopsis^) (Fig. 3) and Limnotrochus^) 

 both crystalline style-sac and spiral caecum are co-existent. 



The presence of a spiral caecum is undoubtedly a primitive feature, 

 being characteristic of the stomach of Pleurotomaria and other 

 Diotocardia, and also, as maintained by Woodward by its probable 



1) J. E. S. MooHE, The Molluscs of the Great African Lakes. Quart. 

 Journ. Micr. Sc, Vol. 42, 1899, p. 190. 



2) Miss DiGBY, On Limnotrochus and Chytra. Journ. Linn. Soc. 

 Zool., Vol. 28, 1902, p. 434. 



3) Martin F. Woodward, The Anatom}- of Pleurotomaria Beyrichii 

 HiLG. Quar. Journ. Micr. Sc, Vol. 44, 1901, p. 237 — 239. 



