380 ESSENTIALS OF ZOOLCKiY 



tory thread, the byssus. In most clams the glochidia are discharged 

 to the outside through the dorsal siphon. They fall to the floor of 

 the river, pond, or lake, and lie with their jaws agape, or snap 

 their jaws on any object. If the soft filament of a fish's gill or a 

 fin of the fish comes in contact Avith the glochidium, it will close 

 down upon it and remain attached if the fish is the suitable host 

 for the particular species of clam. The tissues injured due to the 

 attachment of glochidia produce by proliferation new cells which 

 group up around and eventually cover the parasites. Thus a cyst 

 is produced about the glochidium and within this structure the 

 larval clam undergoes metamorphosis. It shortly breaks loose from 

 its host, drops to the stream or pond bed, and leads an independent 

 life. The rapid dissemination of mussels in a river system can be 

 accounted for by the movements of tfeeir fish-hosts. 



THE SNAIL 



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 

 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 gyriym 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, 

 exposed habitat. Movements of most land snails appear to be co-* 

 incident with moisture rather than darkness. Preceding prolonged 

 periods of cold, land snails may move to protected places, such as 

 beneath dead logs, dense mats of humus, crannies in or under 



