IV.] THE COMMON SNAIL. 279 



mouth, a thickened zone incomplete ventrally. The sur- 

 face of the body becomes ciliated and especially so this 

 zone, whereupon there results a rapid rotation of the embryo 

 within its albuminous investment. This being so, the zone 

 in question is termed the trochal ridge, the larva possessed 

 of it being said to be in the trochosphere stage. This, in 

 turn, gives place to a more advanced veliger stage, so called 

 on account of the changes undergone by the trochal ridge, 

 which now becomes more marked, being produced out into 

 a hood-shaped pre-oral lobe or velum. During this stage 

 the mantle arises as a fold of the body-wall, which, as age 

 advances, takes on the characters of a lung sac. 



During the final stages of larval metamorphosis the left 

 side of the body grows much more rapidly than the right 

 one. Thus it is that the originally bilaterally symmetri- 

 cal larva becomes converted into an asymmetrical adult, 

 a fact which renders clear the displacement of the orifices 

 (other than those of the mouth and pedal gland) and of 

 the organs connected therewith, the suppression of the ex- 

 cretory organ of one side, and the enormous increase in 

 size of the left lobe of the digestive gland, as compared 

 with the smaller right one. 



Very early in the history of the larva the locomotor foot 

 arises, as a median ventral outgrowth between the mouth 

 and anus ; and as the importance of this structure becomes 

 more marked, the cilia of the velum undergo a reduction. 

 Consequent upon these changes the rotatory movements of 

 the animal, so characteristic of the trochosphere, give place 

 to a sluggish creeping motion. The velum itself does not 

 entirely vanish in Lymnaeus, but persists throughout life 

 as a couple of so-called subtentacular lobes which lie im- 

 mediately above the mouth. These are wanting in the 

 " Common Snail." 



