1905.] NATURAL SCIENCES OF PHILADELPHIA. 93 



of the animal. In the typical associations, as in Gregarina, this move- 

 ment is described as passing from the anterior end of the primite to 

 the posterior end of the satellite without the slightest pause. We have 

 here a demonstration that it is not due to endoplasmic movements. 



In general, the conspicuousness of the peristaltic movement depends 

 on the character of the cctosarc. In the stiff, clastic forms this move- 

 ment is by no means common, and when it does occur it generally 

 consists of a single contraction. On the other hand, it is well displayed 

 by the polymorphic species with thin ectosarcs. In one species, 

 Actinocephalus harpali Crawley, I have frequently observed a disposi- 

 tion to rumple the edges of the body, so that they present a scries of 

 scallops. These scallops were seen to undergo slow changes in size 

 and sometimes they moved slowly backward. I take this to be a 

 peristaltic movement, although much slower than is usual. 



The cause throughout is probably contractions of the circular fibres. 

 Further, following the division which I have made, the peristaltic 

 movement comes under the head of shortenings of the longitudinal 

 axis. 



The above are all the movements involving change of shape which 

 can with absolute certainty be credited to contractions of the myocyte. 

 They result in varied and frequently considerable alterations of the 

 contour of the animal, but they can all be placed in two categories : 



(1) Bendings and curvings of the longitudinal axis. 



(2) Shortenings of the longitudinal axis. 



The two may, and doubtless frequently do, take place at once, but 

 apparently the first is by far the more frequent expression of the con- 

 tractility of the myocyte. 



There now remain for consideration a series of phenomena displayed 

 by the polymorphic forms wherein the Polycystidea approximate the 

 conditions normal to the Monocystidea. Of the several species which 

 I have observed, they are best illustrated by Trichorhynchus pulcher 

 Aime Schn. This animal changes shape as readily as Euglena or even 

 Amceha. The anterior margin of the protomerite may present a 

 straight edge, a curve or a long tongue-shaped protrusion. The pos- 

 terior end of the deutomerite may be bluntly rounded, sharply pointed 

 or even bifurcated. The animal may also be so contorted as to lose 

 all semblance to a polycystid gregarine and to present the kind of out- 

 line we associate with Amoeba. These changes take place constantly, 

 and by no means slowly, but always gradually. Stenophora spiroboli 

 Crawley, another polymorphic species, has to be watched very patiently 

 before the true form can be made out. It constantly displays contor- 



