18 



The fibres of tin's muscle are closely packed aud aggre- 

 gated into 12-17 bundles, eacli surrounded»by connective 

 tissue. Througli this connective tissue run the afferent 

 blood spaces of the mantle, gills and mantle skirt. 

 Embryological evidence, derived from Acnu^a, 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 Haliotis and Srissure/Ja among recent Gastro- 

 pods, 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 steejily oblique fibres of the shell- 

 muscle in Patella pull down the edge of the shell by their 

 contraction, while the medianly running inner ones exert 

 downward and inward traction which must gDcatly 

 strengthen resistance to lateral blows. The latter fibres 

 are more numerous in Patella than in more primitive 

 Docoglossa {e.g., Axnuea virginea). 



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 Mantle Skirt is covered dorsally by a layer of 

 columnar epithelium which passes gradually into the fiat 

 pigmented epithelium covering the visceral hump. Peri- 

 pherally it is thrown 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. Near the ext-reme 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. Sr/). The epithelium 



