180 STUDIES IN GENERAL PHYSIOLOGY 
undoubtedly familiar with the fact that many marine animals 
are found only at certain depths in the ocean. Thus far it 
has not been determined what conditions compel these 
animals to behave in this way, Groom and I together have 
shown how the heliotropism of certain pelagic animals must 
lead to periodic depth-migrations.' I can show by several 
examples that gravitation is also of importance in the distri- 
bution of pelagic animals at various depths, and that it is 
this force which compels certain animals to live in the sur- 
face regions of the water. 
Everyone who will watch the animals found about the 
rocks or piles near the surface of the ocean during a quiet 
sea will notice the relatively large proportions of Echino- 
derms. Some of these Echinoderms— such, for example, as 
Cucumaria cucumis which is found in great numbers in the 
Bay of Naples—live near the surface of the water, or not more 
than 80 m. below it. It can be readily shown, however, that 
Cucumaria cucumis is, like the Plasmodia or the Coccinelle, 
compelled to crawl upward on vertical surfaces, and that 
apparently gravitation determines this behavior. Cucumaria 
cucumis has an elongated pentagonal body some 10 or more 
cm. long, carrying at its oral pole radially arranged arbores- 
cent tentacles. Upon each of the five edges are found 
two parallel rows of tiny feet by which the animal is 
enabled to crawl upward, even upon smooth glass plates. If 
these animals are introduced into an aquarium, they creep 
about the bottom until they reach a vertical wall, up which 
they climb to remain at its highest point, and when possible 
immediately under the surface of the water. The animal 
remains permanently in this position and behaves like a 
sessile animal. 
If such a Cucumaria cucumis is permitted to attach itself 
to a vertical glass wall which can be turned about a hori- 
1GROOM UND Logs, Biologisches Centralblatt, Vol. X (1890). 
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