262 



THE RISE OF ANIMAL LIFE 



Fig. 12-12. Gastropoda. Two oyster drills (t/roso/pinx) and one periwinkle {Liftorina) are crawling over a rock en- 

 crusted with barnacles (Balanus). These marine snails are very common on our coasts. 



sexual activity of one species of chiton was 

 influenced by moonlight. It is generally be- 

 lieved that periods of moonlight are pre- 

 ferred by this animal because the tides are 

 low, a condition most favorable for success- 

 ful spawning. Coloration in chitons varies, 

 ranging from tvirquoise and slaty blue to 

 gray and white. Most of the Amphineura 

 are "sby" animals and tend to avoid day- 

 light, although there are some species with 

 furry mantles that do sally forth in the 

 daytime. 



The class Scaphopoda includes several 

 "headless" species, the best known being 

 Dentalkim, the "elephant's tooth." It has a 

 muscular foot which is modified for bur- 

 rowing into the sand and is therefore quite 

 sharply pointed. Its elongated body is en- 

 cased in a tapering shell. Unlike most mol- 



lusks, this animal bears no gills and the 

 mantle alone takes care of respiration. 

 Some of the most interesting and varied 

 forms of the mollusks belong to the class 

 Gastropoda. They range in size from mi- 

 croscopic forms to the large whelks (Fig. 

 12-11). Although most members of this 

 class possess some kind of shell, forms 

 that are entirely without a shell also ap- 

 pear. Some have adapted themselves to 

 terrestrial life as well as the usual aquatic 

 habitat. The most common form is the 

 snail, an animal familiar to almost every- 

 one (Fig. 12-12). The snail is often ob- 

 served wending its way slowly on the 

 leaf of a water plant or along a sandy- 

 bottomed pool. It moves by gliding over a 

 secreted mucus path, using its flattened 

 muscular foot which forms the ventral sur- 



