556 EAYMOXI> PEARL. 



tion all the sets of muscle-fibres other than the longitudinal 

 must be relaxed completely, because as the animal shortens 

 it grows broader and thicker, which would be impossible if 

 the ring, or transverse or dorso-ventral musculature, also con- 

 tracted. The longitudinal musculature is apparently better 

 developed and more effective on the ventral side of the body 

 than on the dorsal, because after very strong stimulation of 

 the anterior end there is a well-marked tendency for the 

 middle portion of the body to be raised and the head some- 

 what curled in under it. This relation is shown diaoram- 

 matically in Fig. 5. This curliug under of the head does not 

 appear to be a specific reaction, but, on the contrary, merely 

 an expression of the fact that the ventral musculature is 

 capable of shortening its side of the body more in maximal 

 contraction than the dorsal side. Jijima (loc. cit., p. 378) 



Fig. 5. — Diagram showiiif^ the appearance in side view of a 

 niaxinialiy contracted planarian. 



finds, from a histological study of the musculature, that the 

 bundles of fibres in the main longitudinal muscle layer are 

 thicker on the ventral than on the dorsal side. 



b. Extension of the Body. — The mechanisms by which 

 extension of a soft-bodied animal is brought about are 

 probably very different in different groups. In the case of 

 the flat-worm extension is produced by the contraction of the 

 circular muscular layers surrounding the body, and of the 

 transverse and dorso-ventral systems of musculature. Probably 

 also the oblique musculature, when present, assists, by its 

 contraction, in the extension of the body. The mechanical 

 necessity for extension of the bodj^, after contraction of these 

 muscles, is readily apparent. If the body, for simplicity's 

 sake, be considered a cylinder, contraction of circular 



