FRESH-WATER PLANAR! AN 163 



The behavior of a planarian is of a 7'eflex type — that is, a stimulus 

 received by a cell on the surface produces an impulse which is conducted 

 along a nerve fiber to a cell in a ganglion or in a nerve cord. This cell 

 in turn sends out an impulse which is transmitted either to muscle 

 cells, which move, or to gland cells, which secrete. In other words, the 

 effect is turned back or refiexed. The cell receiving the stimulus is 

 called a receptor, the ingoing impulse an afferent impulse, the central 

 cell an adjustor, the outgoing impulse an efferent impulse, and the cell 

 whish completes the action — not a nerve cell — an effector. A reflex act 

 may be defined as an act involving these three types of cells or as an 

 act involving an afferent and an efferent impulse in which 

 the latter is conditioned upon the former. 



Planarians are subject to different physiological states, 

 the character of their reactions varying with hunger, fatigue, 

 or nervous excitement. 



190. Regeneration. — A planarian possesses a power of 

 regeneration hardly less developed than that of the hydra. 

 Pieces of their bodies may also be grafted together without 

 great difficulty. 



A noteworthy fact is that whenever a new body is 

 regenerated from a piece, a head is developed on that . ^^°- ''^•~ 

 margin of the piece which was nearest the head in the illustrate the 

 animal from which it came, while a new tail is developed metabolic 

 on the opposite margin. The explanation of this was for planarian. 

 a long time obscure but has been furnished by recent experi- The change 

 ments. These show that in an animal possessing a head the black line 

 and a tail, as do planarians, there is a gradient in metabolic shows the 

 activity extending from near the anterior end to the grees^ of meta- 

 posterior end. The rate of metabolism is greatest at the boiic activity 

 anterior end of this gradient and decreases gradually from levels, such as 

 this end to the other (Fig. 75). Thus it is that any a, b, c. The 

 fragment will differ in the metabolic activity of its different c o u r s e , n o t 

 portions corresponding to their position with respect to the confined to the 



■ 1 J • , r^ J.1 r iu • T_ medianline 



axiat gradient. Consequently, from the margin where i-^^ extends 

 metabolic activity is greatest a head will develop, and from from one side 

 the other margin a tail. This conception of an axial meta- 

 bolic gradient, proposed first by Child, can also be applied in explaining 

 how reproduction in some worms may occur by transverse fission. It is 

 assumed that when the animal gets so long that the gradient becomes 

 exceedingly gradual, the posterior portion of the body escapes from the 

 dominance of the anterior portion and a new center of metabolic activity, 

 or another maximum in a new axial gradient, is established. Just in front 

 of this center appears the constriction which divides the body into two 

 parts. 



