Japanese Freshwater Triclads. 55 



the rliytlimical Seating of the margins of the Ijody, aided perhaps 

 very httle by the cilia of the ventral surface. Also they are able 

 to make progress through the water by means of muscular move- 

 ments of the body as a whole, or of undulations of its expanded 

 margins, tiie ciliai'y activity taking no necessary part in this form 

 of movement. In the freshwater and marine Triclads, the cilia 

 have become much diminished in comparison with the Rhab- 

 docoeles, and are not numerous and strong enough to support and 

 move the disproportionately heavier body freely through the w^ater. 

 The muscular movements of the body or its lateral extensions are 

 not so highly developed as those of the Polyclads. In consequence, 

 the Triclads are not capable of performing free movements in the 

 water without contact with any solid body. Judging from the 

 above, the Triclads seem to occupy a somewhat intermediate posi- 

 tion in respect to locomotor phenomena betAveen the Khabdocoeles 

 and Polyclads. 



Non-locomotor Movements. — Under this description of move- 

 ments will l)e included tlie phenomena of contraction, extension, 

 feelijig movements, extrusion of the pharynx, etc. 



The feature of the worm when resting or stimulating is quite 

 different from its featui'e in the active motion. The body groAvs 

 shorter, wider and thicker. The oi'dinaiy contour of the head is 

 almost entirely lost, and the end is evenly rounded like that at 

 the posterioi'. The lateral margins of the body form a wavy line 

 instead of the straight one of the active condition. This 

 shortening of the body lengthwise is usually designated by the term 

 "Contraction of the body." In the condition most contracted 

 longitudinally after stimulation, many individuals decrease one- 

 half or one-third the bod^'-length in the full extension. This is 

 produced by the contraction of the longitudinal muscular fibres, 

 Avhich are apparently better developed on the ventral than on the 

 dorsal side. In this case all the sets of muscular fibres other than 

 the longitudinal must, of course, be relaxed completely, because as 

 the animal shortens it grows broader and thicker, which would be 

 impossible if the circular, or transverse, or dor:-o-ventral nuiscular 

 übres also contracted. 



