MOVEMENTS, ETC., OF FRESH-WATER PLAXARIAXS. 557 



muscles must cause it to lengthen, wliile witli the form of 

 body which really exists in the flat-worm, the contraction of 

 the well-developed dorso-ventral fibres must bring about the 

 flattening seen in the fully extended gliding auimal. 



Probably by far the most important sets of muscle-fibres 

 for producing the general extension are the dorso-ventral and 

 circular. It is to be noted that contractions of any of the 

 sets of fibres may take place in localised regions, producing 

 extensions or contractions of that region, according to the set 

 affected. The sensory, or " feeling " movements of the head 

 are brought about in this way. 



The extension, and probably also in large part the extrusion 

 of the pharynx is brought about by contraction of its well- 

 developed circular musculature. 



In the case of the marine mollusc, A ply si a li niacin a, 

 Jordan (: 01, pp. 11 — 15) has recently shown that extension is 

 brought about in an entirely different manner. It results 

 from passage of fluid from vesicles in the skin into the spaces 

 in the body parenchyma when the body muscles are relaxed. 

 When the animal conti^acts this fluid (blood) is pressed out 

 into the vesicles, which become very much extended and 

 swollen ; then, when the muscles are relaxed, the elasticity of 

 the walls of the vesicles forces the fluid back into the body, 

 and thus causes its extension. As a result of this method of 

 expansion it is possible to kill the animal fully extended by 

 the use of such poisons as cocaine. 



It is not unusual to consider the fully extended condition 

 of such an organism as a flat-worm as one of approximate 

 relaxation. Instead of this, it is, in fact, a condition in which 

 a certain part of the musculature is in a state of well-marked 

 tonic contraction. This furnishes a reason for the fact that 

 it is impossible to kill these animals in a completely extended 

 condition by the use of poisons which tend to produce a 

 relaxation of the muscles. Under these circumstances the 

 animals take on the typical relaxed form, which is quite 

 different from that of extension. 



c. Rest. — Inasmuch as a very large, if not the larger 



VOL. 46, PART 4. NEW SERIES. O 



