90 PROCEEDINGS OF THE ACADEMY OF [Jan., 



Stenophora jidipusiUi Leidy, tend to maintain a certain definite shape. 

 Changes of this shape may be extensive, but are usually of short dura- 

 tion. Further, the contortions are of such a character that the true 

 form is by no means disguised, and the resumption of this true form is 

 always a very sudden process. We may conclude from this that the 

 ectosarc is stiff and elastic. Under the force of the contractile ele- 

 ments it may suffer contortion, but as soon as the force is released 

 the proper form is rcsvmied with a sudden jerk. The comparison may 

 be made with a hollow india-rubber ball, or, perhaps better, it may be 

 said that these grcgarines behave as if their ectosarc were composed 

 of india-rubber. They may further be compared with such ciliates 

 as ParamcEcium, at least in so far as regards the sudden resumption of 

 the normal contour after a deformation. 



On the other hand, gregarines like Trichorhynchus pulcher Aime 

 Schn. are highly polymorphic. Different individuals present c^uite 

 different outlines, and these outlines are subject to continual and 

 extensive changes. Their movements may not inaptly be termed 

 amoeboid. All polycystid gregarines possess what may be designated 

 as a typical outline, but it is often difficult, in these polymorphic 

 species, to determine what this typical outline may be. Their ecto- 

 sarc, while extremely flexible and extensible, does not appear to possess 

 any elasticity. In consequence, deformation may be carried to such 

 an extent as to render the typical form wholly unrecognizable, nor is 

 there ever seen that sudden resumption of the typical form so often 

 displayed by such species as Stenophora julipusilli. 



This difference appears to be due mainly to the sarcocyte. Usually, 

 in these polymorphic animals, the ectosarc consists of epicyte alone. 

 Further, the epicyte may be thin. The facts, then, are in accord with 

 what we should expect on mechanical principles. The elastic species 

 possess a sarcocyte; the polymorphic do not. It may therefore be 

 inferred that the elasticity is due to the sarcocyte, and confirmatory 

 evidence is furnished by the Monocystidea. These animals, which are 

 frequently highly polymorphic, appear generally to lack a sarcocyte. 

 The rule, however, is not absolute, since Stenophora iCnemidospora) 

 spiroholi Crawley, the protomerite of which is very flexible, possesses 

 a thick sarcocyte. 



Obviously, the ability of a gregarine to display changes of form 

 depends upon two factors.^ These are the flexibility of its ectosarc 

 and the power of the contractile elements. It has been shown that 



' There is possibly a third factor, as will be pointed out below. 



