262 



ZOOLOGY 



pods in which the nerye loop is not twisted. The head usually bears two pairs of 

 tentacles. The sexes are united in the same individual. The most important 

 of these are the Pulmonata or air breathing Gasteropoda, some of which are 

 terrestrial and others aquatic. Of the terrestrial snails the genus Helix (Pig. 121) 



is the most widely distributed. Its variability is such that between three and 



Fig. 122. 



Fig. 122. Ltmax ^ovus, a Slug. From Binney's Gould. 



Questions on the figure. — How do the slugs differ from the other gasteropods ? 

 In what external respects do they appear similar to them? Compare all the 

 figures of slugs you may be able to find. 



Fig. 123. 



Fig. 123. Pearly Nautilus. From Nicholson. *;. eye; /i, hood, a muscular portion of the foot 

 which protects the softer parts; 5, siphon; se, septa, separating the successive chambers of the shell; 

 sp, siphuncle; f, tentacles. 



Questions on the figure. — How does this shell compare with those of the 



gasteropods? What is considered to be the homology of the tentacles or arms in 



cephalopods? What is the siphuncle? What is the character of the eye in 

 Nautilus? 



four thousand species have been described. Limax (Fig. 122) is a pulmonate form 

 in which the shell is practically wanting. It is especially destructive to certain 

 types of plants as it is a voracious vegetable feeder. The aquatic pulmonates are 

 represented by the "pond-snail" {Limnaa), and by Planorbis, a snail whose coils 

 are in one plane, presenting a helix rather than a spiral. 



