PHYLUM MOLLUSCA 



237 



annelids is a matter for conjecture since some morpboJogists regard 

 this similarity in larval forms as an example of adaptive parallelism 

 in a similar type of environment. Certainly morphological evi- 

 dence shows a close relationship. 



THE SNAIL 



(Detailed description based on Helix) 



Habitat and Behavior 



Snails occupy a variety of habitats. They occur abundantly in 

 fresh water, salt water, brackish water, and thermal springs; they 

 live in the arid sections of the country and occur abundantly in the 

 tropics where certain arboreal forms are found. Some species belong- 

 ing to the genera Caecilianella and Helix live underground, feeding 



[Mesodermal 



band 



EsopbaS 

 Head kidney. 

 Qtocfst 



Mesenchyme 



Apical or^an 

 Eye 



Stomach 



Preoral 

 ciliated 

 ring 



Blastocoelz 



_, ■MS 



Rnal vesicle 



Apical or^an 



Endoderm 

 Embryonic mMcle 



Prototroch 



Mesoderm 



Jelotroch 



Fig. 129. — A, Trochophore larva of Eupomatus (a polychaete annelid), side 

 view. (After Shearer.) B, Veliger larva of Patella (a marine snail) frontal sec- 

 tion. (After Patten. (Drawn by Joanne Moore.) 



on roots of plants ; many other species live deeply embedded in moist 

 humus. Certain species, such as Helix hortensis and Helix aspersa, 

 excavate holes in rocks and live in them. Although most snails are 

 not tolerant to extremes of cold, Vitrina glacialis lives in the Alps 

 above the timberline where the rocks are covered with snow most of 

 the year; even some of our fresh-water snails in this country, such 

 as Lymnaea palustris, Physa gyrina and Helisoma trivolvis, when 

 frozen gradually, can live at least several weeks in solid cakes of ice. 

 Land snails are most active either during a light rain or immedi- 

 ately following. In heavily shaded woodlands where surface moisture 

 prevails, snails are active during the day as well as at night. The 

 same species of snail that exhibits both diurnal and nocturnal ac- 

 tivity in the woodland may show only nocturnal activity in an open, 



