THE SLUG 



219 



belongs to a type in which the spire has become almost obsolete. 

 It is still represented, however, in a rudimentary way at the 

 left of the aperture (Fig. 208). 

 The modified shape is asso- 

 ciated with the habit the mol- 

 lusk has of lying, aperture 

 downward, close to the rock or 

 another shell. By this means 

 the animal is better protected. 

 The " deck " is a plate which 

 has been developed internally 

 to help hold the animal in the shell. 

 The limpets are modified sea-snails 



Fig. 208. — Crepidula, the boat shell. 

 Two-thirds nat. size. Photo, by 

 W. H. C. P. 



They are bilaterally 

 symmetrical and are cov- 

 ered by a flat, conical shell 

 which is applied closely to 

 the rock all around the 

 base in such a way as to 

 protect the animal within. 

 In some species (genus 

 Fissurella ^) there is an 

 opening at the apex of the 

 shell through which the 

 water is expelled that has 

 passed over the gills. 



Our common Eastern rook 



Fio. 209. — Patella vulgata, seen from the limpet is not perforated ; it 

 ventral side. /, foot ; g.l, circlet of gill 

 lamellae ; m.e, edge of the mantle ; mu, at- 

 tachment muscle ; si, slits in the attach- 

 mentmuscle; sh, shell; v, efferent branchial 

 vessel: v', aorta; ye, smaller vessels. From 

 the "Cambridge Natural History." 



is known as Acmcea.'^ In 



' A little slit or fissure. 

 2 akmaios, in full bloom, 

 maturity. 



