18 



TliG fibres of thfs muscle are closely packed and ao-ore- 

 gated into 12-17 bundles, each surrounded by connective 

 tissue. Through this connective tissue run the afferent 

 blood spaces of the mantle, gills and mantle skirt. 

 Embryological evidence, derived from Acnuea, shows us 

 that this type of muscle has arisen by enlargement back- 

 wards of a pair of lateral muscles. Such a paired condition 

 exists only in HaJioiis and Schxurella among* recent Gastro- 

 })()(ls, and in the former the left member of the pair is 

 almost vestigial. The columellar muscle of the typical 

 Prosobranch is the modified right member of the ancestral 

 pair. The outer more stee])ly oblique fibres of the shell- 

 muscle in Pdfelhi pull down the edge of the shell l)v their 

 contraction, while the medianly running inner ones exert 

 downward and inward traction which must greatly 

 strengthen resistance to lateral bloM's. The latter fibres 

 are more numerous in Patella tlian in moie primitive 

 Doeoglossa {e.g., Acnuea virgined). 



The Pallial Muscles attach the mantle skirt to the 

 shell all round and will be described in the account of the 

 mantle which follows. Their insertion into the shell has 

 already been mentioned. 



The Maktle Skirt is covered dorsally by a layer of 

 columnar epithelium Avhicli passes gradually into the flat 

 pigmented epithelium covering the visceral hump. Peri- 

 pherally it is throwm into a large number of folds (fig. 8j 

 parallel to the free edge, the cells on the crests of these 

 folds being very much elongated. Xear the extreme edge 

 of the mantle on the dorsal side the epithelium is not 

 folded in this way. There is a broad pigmented band 

 outside and concentric with the shell muscle and a 

 narrower and less continuous band of the same character 

 a few cells from the free edge of the mantle, the mantle 

 edge itself is also pigmented (fig. 8a). TJie epithelium 



