Structural Variation among the Difflugian Rhizopods. 235 



physical forces combine, probably in every structure called or- 

 ganic, to build up the tissues of which the animal and vegetable 

 kingdoms are composed. 



But another important fact bearing on the series of forms re- 

 presented in figs. 26-33 must now be noticed. In these the 

 tests are sometimes so compressed as to give the aperture, when 

 the structure rests on one side, the undulating appearance exhi- 

 bited more particularly in figs. 27, 29, & 30. But more fre- 

 quently, the tests are not compressed, and the aperture pre- 

 sents the ordinary circular or nearly circular outline. Whilst 

 endeavouring to gain a clear end-view of one of the undulating 

 apertures, namely the one shown in fig. 30, 1 found it to be closed 

 as shown in fig. 30 b, and, further, the sarcode-mass gathered 

 into a spherical mass, occupying the middle half only of the test, 

 the transparent texture of which, in the specimen referred to, 

 enabled me to discover that this mass was prevented from es- 

 caping by a well-defined membranous diaphragm which seemed 

 to be attached around the interior of the wall of the test a little in 

 front of its broadest portion . The true significance of the spherical 

 body would, however, probably have escaped me, but for the oc- 

 currence of a hyaline crescentic space between its anterior convex 

 part and the interior of the diaphragm, the convexity of which 

 was also directed outwards. 



Here then it became evident that a process of encystation 

 precisely similar to that described by me as occurring in Amoeba 

 ('Annals,' November 1863, p. 336) had taken place in a testa- 

 ceous form, and that the test is neither the analogue nor the re- 

 presentative of the cyst of the naked forms, but must be regarded 

 as an entirely distinct portion of the structure. 



Now, how far the peculiar external features witnessed in the 

 series of tests above alluded to may be associated with the de- 

 crease of the ordinary process of nutrition incident on encysta- 

 tion, and may thus tend to render the chitinous material of the 

 test free to be acted on by purely chemical or physical forces, I 

 am at present unable to say. It must not be forgotten, however, 

 that the complete withdrawal of the body of the animal into the 

 interior of the test, and its envelopment by the cyst, would 

 necessarily prevent the diffusion of any portion of the sarcode- 

 substance over the external surface of the test, and hence ex- 

 ternal influences would act directly on the test. But the repeated 

 occurrence of similar tests with open apertures, and with the 

 animal, at all events as yet, unimpeded by the cyst, seems clearly 

 to indicate that the one set of conditions is not necessarily in- 

 compatible with the other. On the other hand, I am able 

 to state that the singular characters presented by the tests 

 and the process of encystation referred to are not confined to a 



16* 



