148 



THE EVOLUTION THEORY 



genus BaetiSy have the instinct to press their smooth broad bodies, 

 with large-eyed head, close to the brook pebbles on which they sit. 

 They are exactly like these in colour, and thus they lurk almost 

 invisible, until a victim comes within their reach, when they throw 

 themselves upon it with a bound. The third group, with the typical 

 genus Ephemera, follows its instinct to dig deep tubes in the mud 

 at the bottom of the water, and to lurk in these for their prey. We 

 have thus within this small group of Day-flies three distinct 

 modifications of the food-instinct, which differ essentially from one 



another, are made up of quite 

 different combinations of actions, 

 and, consequently, must have their 

 foundation in essentially different 

 directive brain-mechanisms. All 

 these cases have only one feature 

 in common ; the animals all throw 

 themselves upon their prey as soon 

 as they are near enough. 



But even this common feature 

 is not everywhere part of the 

 food-instinct. The sea-cucumber 

 (Cucumarla) (Fig. 31), according 

 to the observations made on it 

 by Eisig in the Aquarium of the 

 Zoological Station at Naples, gets 

 its food in the following manner. 

 The animal sits half or entirely 

 erect on a projecting piece of rock 

 and unfolds its ten tree-like ten- 



Fig. 31. Sea-cucumber (Cucumaria), with tacles which SUITOUnd the mouth, 

 expanded tentacles (a), and protruded w i i i -, i mi jf P 



tube-feet (b) ; after Ludwig. J-nese aie uiancnea, ana na\ e quite 



the effect of little tufts of sea- 

 weed. They are probably taken for such by many minute animals ; 

 for larvae of all kinds, Infusorians, Rotifers and worms settle down 

 on them. But the sea-cucumber bends inwards first one tentacle and 

 then another, so slowly as barely to be noticeable, brings the point to 

 its mouth, lets it glide gradually deeper into the gullet, until the 

 whole tentacle is within, and after a time draws it out again equally 

 slowly and unfolds it anew. Obviously it wipes the tentacle inside 

 the gullet, and retains everything living that was upon it. This 

 performance is repeated continually, day and night, and it is usually 

 the only externally visible sign of life which the animal displays. 



