542 IJAYMOXD PEARL. 



This position is shown in Fig. '^^. As will be brouG^lit ont 

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

 and appai'ently 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 (''tastende Bewegnngen ") 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 np and down and extended 

 and retracted. These '^feeling" movements of the head 

 region are usually ver}^ slight, and escape notice except 

 under close observation. When the organism is much 

 stirred up, liowever, they mny become quite apparent. 

 Their purpose is evidently to increase the chances of re- 

 ceiving stimuli, so that any stimulus in the neighbourhood 



Fig. o. — Diui^ramniatic side view of a f^lidiiitr |)laiiaiiaii. 



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 the auricles are fully extended and 

 raised. I do not think that this marked sensoiy 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 woulil not appear 

 that such an organism as the flat-worm runs as great risk 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 studied, no trace was 

 observed of a reaction adapted to the purpose of getting the 

 organism out of dangerous surroundings, but its only specific 



