FRESH-WATER PLANARIAN 165 



The behavior of a planarian is of a reflex 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 gangUon 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 reflexed. The cell receiving the stimulus is 

 called a sensory neuron, the ingoing impulse an afferent impulse, the 

 central cell a motor neuron, the outgoing impulse an efferent impulse, and 

 the cell which 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. 



192. 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 ijjf'g';'^^^""^ 

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

 animal from which it came, while a new tail is developed '^^}^}'?}^l 



- gradient in a 



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



a long time obscure but has been furnished by recent experi- J^^J/^j^^^^^jf 



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 



' • J J. +1, varying de- 



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

 posterior end. The rate of metaboHsm is greatest at the b o Hc activity 



J ,, J. at different 



anterior end of this gradient and decreases gradually trom ig^eis, such as 



this end to the other (Fig. 80). Thus it is that any «,^^f'. c-^ The 



fragment will differ in the metaboHc 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- o , ,. ,1 -1 median line 



axial gradient. Consequently, from the margin where ^ut extends 

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



.1 rr^i • J • J! . 1 , to the other. 



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

 bohc 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. 



