Spinning Shigs and Snaih. 42i> 



iioiuislicil imlividuals containing inir^li moisture only aflc)i)t 

 this course when one has made the distance from tlie groun 1 

 too <;reat, or when liijht is thrown on them from below, wiiilo 

 animals less favourably equipped in these respects make the 

 return journey relatively often. 



Tiie way in which the animals behave upon an isolated 

 leaf, which is brilliantly lighted or warmed from al>ove, or 

 upon which a strong current of air is suddenly directed, shows 

 us that they are not very unaccustomed to the route througli 

 the air; if the expression " deliberation '' be employed with 

 the necessary limitation, it is here apropos. At least it may 

 be said that letting themselves down upon a thread of mucus 

 is to be numbered among the normal faculties of the 

 animals. 



The circumstance that the animals do not try to escape 

 upwards on the thread by which the leaf which serves as an 

 experimental table is .su|)ported, is simply due to the fact that 

 the infl'ience which disturbs the animals makes itself felt 

 from above. When I applied the source of light and warmth 

 (in the form of an electric incandescent bulb) to the under 

 side of the leaf, I succeeded in several instances in causing- 

 the slugs to take to Hight in an upward direction. It is true 

 that, when they come too near to the lamp, the animals for 

 the most part simply let themselves drop. 



Under normal conditions also the land-snails seek their 

 hiding-place in a downward direction, when light or warmth 

 becomes too troublesome to them. 



The hardened thread of mucus that has become useless on 

 the crawling back of the animal is not, as Ballerstedt sup- 

 poses, sucked up again by the slug's mucous membrane, but 

 is thrust to the hinder end of the foot owing to the movements 

 of the latter ; here it is stuck together by the freshly excreted 

 slime, and is subsequently left behind when the animal has 

 again reached a Hrm support. 



In the case of the water-snails two forma of thread-drawino- 

 can be distinguished, since the water enables the animals to 

 ascend as well as to let themselves down. While the animals 

 when descending in jerks twist hither and thither like the 

 slugs and turn rciund on their axis, in consequence of 

 which the thread (under the microscope) appears sj)irally 

 twisted, climbing up almost always takes place quietly. In 

 descending the animal has the sole of the foot turned down- 

 wards longitudiTially so as to form a kind of groove, and 

 slightly arched trom front to rear, so that the head occupies 

 an almost horizontal position. Climbing up takes place mure 

 continuously i\\v\ somewhat slowly. The thread may attain 



