LYMN^EKLE OF NORTH AMERICA. 37 



"Undoubtedly a snail sometimes takes a roundabout path to the 

 surface, but it is fully as liable to arrive at the surface in a short inter- 

 val of time as some other individual that has scarcely twice its own 

 length to go. The important factor appears not to be so much the dis- 

 tance to be traveled as the animal's need of oxygen and its response 

 to food stimulus. The 'need of oxygen' does not explain how breath- 

 ing movements occur, but it may possibly give the clue to the cause. 



"That different snails take in varying amounts of air may be 

 shown by the number of bubbles they may be forced to give up after 

 having been to the surface to breathe. A variation of 1 to 14 bubbles 

 was demonstrated by experiments with the snails under consideration. 

 Naturally a snail who has taken in 14 bubbles can remain under water 

 longer than an individual who has taken in but one, provided other 

 conditions are equal. The rate at which a snail travels to the surface 

 is decidedly influenced also by the pasturage through which the latter 

 passes on its way. 



"Table VII summarizes 363 observations on 12 different snails, 

 showing an average interval of 25 minutes. The actual intervals varied 

 without uniformity from 15 seconds to several hours. For instance, 

 on July 6, No. 3 did not change its position at the bottom of the dish 

 from 10 a. m. to 5 p. m., when the observation closed. How much 

 greater the actual interval may have been is not known. 



"The twelve snails whose movements are tabulated above were all 

 in the same dish, with an equal opportunity to reach the surface by any 

 course, except No. 40 and No. 41, which were in a smaller dish to- 

 gether with a numerous company of Tubifex rivulorum. It will be 

 noticed that No. 40 and No. 41 came to the surface at shorter inter- 

 vals than the others. This was probably due not to the smaller size of 

 the dish, but rather to the higher temperature of the water. It is un- 

 doubtedly true, as will be shown from evidence given later, that these 

 snails obtain part of their oxygen through the skin from the water 

 itself — a factor that must be borne in mind when considering the inter- 

 vals between their visits to atmospheric air. 



"The amount of visible breathing does not seem to be closely 

 correlated with the amount of activity which the animal exhibits. For 

 example, the details of the movements summarized in Table VII clearly 

 show that snails which travel most are not necessarily the ones which 

 come most frequently to the surface to breathe. In other words, de- 

 pendence on atmospheric air does not seem to be a constant factor of 

 paramount importance. 



