MOLLUSCA 



26l 



pods in which the nerve 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 Gasteropods, some of which are 

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

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



FIG. 122. 



FIG. 122. Limax flavus, 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. 



e' 



FIG. 123. Pearly Nautilus. From Nicholson, e, eye; h, hood, a muscular portion of the foot 

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

 sp, siphuncle; t, 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? 



three and four thousand species have been described. Limax (Fig. 122) is a pul- 

 monate 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 pul- 

 monates are represented by the "pond-snail" (Limncea), and by Planorbis, a snail 

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



