SNAILS AND SLUGS 323 



of danger. Although the hump and shell still retain a certain 

 amount of twisting they have been flattened out to a large extent, 

 and the shell no longer serves as a refuge. This, however, is 

 made up for by the immense size of the foot, by which the animal 

 can adhere firmly to the rock, at the same time pulling the shell 

 •down so as to cover the exposed parts. 



In the much smaller Key-hole Limpet (Fissurella Grcsca) of 

 the Mediterranean the visceral hump is completely flattened out, 

 and the shell is conical, with a hole at the apex communicating 

 with the mantle-cavity. It possesses two gills, &c., like the 

 Ormer. ^ 



In John Knox's Limpet {Acmcea testudinalis), not uncommon 

 on certain parts of the British coast, still further changes have 

 taken place, for there is only one auricle and a single gill. Nor 

 ■does the intestine pass through the heart, as is the case in the 

 shield - gilled forms so far mentioned. The Common Limpet 

 {Patella vulgata) agrees with this species in most respects, but 

 has lost both the gills, at least as functional breathing -organs. 

 If the small mantle-cavity lying above the neck be cut open the 

 ■end of the intestine will be seen projecting into it, and on each 

 side of this the opening of a kidney. On the floor of the cavity 

 are two little orange-coloured projections, examination of which 

 as to structure and nerve-supply shows that each represents the 

 vestige of a gill covered by its water-testing organ (osphradium). 

 The Limpet, however, does possess gills, though of another kind, 

 which are seen as a large number of flattened plates running right 

 round the body well above the foot and overhung by the mantle 

 skirt, which is a well -developed continuous flap. Since these gills 

 are not the equivalents of the ordinary plume-like gills characteristic 

 ■of Molluscs they are termed secondary gills. This use for the 

 word secondary is a common one in zoology. 



At first sight a Limpet, with its simple conical shell, might 

 be taken for a very primitive animal. If it were so, however, 

 the mantle-cavity, with its related organs, would be at the hind 

 •end of the body instead of in front, and the nerve-loop of the 

 nervous system would not be, as it is, 8-shaped. These con- 

 siderations, and comparison with other forms, would lead to the 

 conclusion that the apparent simplicity is secondary, and that 

 the Limpet's ancestors were forms with spirally coiled visceral 

 hump and shell. A very interesting confirmation of this con- 



