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. 1 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 30 m. below it. It can be readily shown, however, that 

 Cucumaria cucumis is, like the Plasmodia or the Coccinellae, 

 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- 



i GROOM UND LOEB, Biologisches Centralblatt, Vol. X (1890). 



