542 RAYMOND PEARL. 



This position is shown in Fig. o. As will be brought out 

 later, the head is a particularly sensitive portion of the bod}', 

 and apparently its elevation is related to its sensory function, 

 in that it practically brings the head into close relation with 

 a large environmental field. The head is not held in a fixed 

 raised position, but is in constant though slight movement 

 whenever the animal as a whole is moving. These "feeling" 

 movements ('Hastende Bewegungen ") of the head are very 

 characteristic. The head as a whole is raised and lowered, 

 and turned from side to side, while at the same time the 

 antero-lateral margins are moved up and down and extended 

 and retracted. These "feeling" movements of the head 

 region are usually very slight, and escape notice except 

 under close observation. When the organism is much 

 stirred up, however, tliey may become quite apparent. 

 Their purpose is evidently to increase the chances of re- 

 ceiving stimuli, so that any stimulus in the neighbourhood 



Fig. 3. — Diuj^rammalic side view of a gliding plaiiaiiaii. 



may be quickly received. Constantly different sensory 

 surfaces are presented to the environment. The head region 

 acts in movement as a single great tentacle-like organ which 

 is constantly testing the environment as the animal pro- 

 ceeds. At the same time tlie auricles are fully extended and 

 raised. I do not think that this marked sensory activity 

 functions so much for the protection of the organism against 

 harmful environmental influences as it does to give prompt 

 notice of useful stimuli, — for example, stimuli due to the 

 presence of food material. In general it would not appear 

 that such an organism as the flat-worm runs as great i*isk of 

 elimination from enemies as it does from not finding food 

 material for its own support. In the ctenophore Mnemiop- 

 sis Leidyi, whose reactions I have studioLl, no trace was 

 observed of a reaction adapted to the purpose of getting the 

 organism out of dangerous surroundings, but its only specific 



