212 



ZOOLOGY 



alierrant gasteropod. Other land gasteropods — the snails — 

 have an evident shell. In Limax the shell is reduced to a thin, 

 horny plate, embedded ui the mantle. Between the condition 



seen in the snail and that 



I tc.d in Limax there are inter- 



^P-ir^^]„„ mediate conditions, some 

 in which the large shell is 

 partly covered by the man- 

 tle, and others in which the 

 shell has become reduced in 

 size. In alhes of Limax — 

 in a genus called Arioji — 

 the shell is reduced to a few 

 calcareous grains. An ex- 

 actly similar series in the 

 degeneration of the shell is 

 found in certain marine 

 snails, illustrated in Figure 

 197. The beginnings of this 

 process of covering the shell 

 are seen in many gastero- 

 pods in which the mantle edges may protrude beyond the lips 

 of the aperture, and are folded back over the outer surface of 

 the shell. A mere developed condition is seen in species like 

 Aplysia, in which the mantle is permanentl.y turned over the 

 back.' In Limax the turned-up edges of the mantle have 

 permanently grown together. The turning back of the mantle 

 seems to afford additional protection to the visceral mass. 

 But after the complete overgrowth of the mantle the shell is 

 useless, and consequently degenerates. 



1 Fig. 198 D. 



Fig. 198. — Illustrating the gradual cov- 

 ering of the .shell {sli) in certain Opistho- 

 branchs by the epipodia (ep) and 

 mantle ; c.d, cephalic disc. Drawn to 

 various scales. A, Haniinea ; B, Sca- 

 Ijhandcr ; C, Aplustruni ; D, Aply.sia ; 

 i?, Philine. From Cooke, " Mollusca." 



