HEDGE AND DITCH 167 



was removed at each bite. In all the stony brooks of the 

 district the freshwater limpet (another objectionable name, 

 naturalists call it Ancylus) abounds ; let us collect a few 

 and put them in a separate globe. The biting track of 

 an Ancylus is quite different from that of a pond-snail", 

 it zigzags, as if the creature bent its head by turns as far 

 to the right and as far to the left as it would do without 

 moving its whole body, and in each of the sidelong clear- 

 ings you can distinguish the separate bites. Though pond- 

 snails subsist mainly on vegetable food, they enjoy an 

 occasional taste of flesh, and may now and then be seen 

 devouring a dead minnow or some other carcase which 

 fortune has put within their reach. 



Pond-snails do not hibernate, but move about and feed 

 all through the winter ; in summer droughts, when the 

 water dries up, they bury themselves in the mud. 



The sexes are united, and every pond-snail is both male 

 and female. During the summer months they lay their 

 eggs many together in gelatinous strips, which are made 

 fast to stones, or to the stems and leaves of aquatic plants ; 

 in an aquarium the eggs are often glued to the glass. 

 Owing to their transparency, these egg-masses are very 

 convenient for microscopic study ; like the egg-ropes of 

 Chironomus or a caddis-fly, they can be examined alive 

 time after time. Within each egg-membrane there can 

 be seen a globular mass of yolk ; after a little while trans- 

 parent cells appear on one side, the first cells of the future 

 body. When the little snail begins to take a definite 

 shape, it is found to be encircled by a ciliated girdle, which 

 lies in front of the mouth, and sets up a steady rotation 

 of the embryo within the egg-membrane. In a later stage 

 of development the simple girdle becomes drawn out into 

 paired lobes, which project on either side. All this is just 

 the same in any larval sea-snail, except that the ciliated 

 girdle there becomes an effective organ of locomotion, 

 enabling the footless and finless larva to propel itself 

 through the sea, and, with the help of currents, to trans- 



