14 OF THE RHIZOPODA IN GENERAL. 



that of the former contains a much larger proportion of the granules and vacuoles, and seems 

 less endowed with self-activity, though very easily put into passive motion. This differentia- 

 tion, which is sometimes scarcely perceptible, in other cases proceeds so far that the coloured, 

 granular, vacuolated sarcode is surrounded by a perfectly transparent envelope ; and the two 

 portions thus distinguished may be appropriately designated (after Dr. Strethill Wright) as 

 the endosarc and the ectosarc. 



6. In the cases in which this differentation has proceeded furthest, so that the body of the 

 Rhizopod bears the strongest resemblance to an ordinary " cell " (as is the case with Amoeba 

 and its allies (^ 21), a nucleus may be distinctly traced ; in those, on the other hand, in which 

 the original protoplasmic condition is most completely retained (as seems to be the case with 

 Gromia, and with the Foraminifera generally), no nucleus can be distinguished. The same 

 appears to be true of the peculiar contractile vesicle, which may be regarded as a vacuole 

 with a defined wall ; for this seems to be always present in the " ectosarc " of those Rhizopods 

 which have that portion of the body most differentiated from the " endosarc," whilst it cannot 

 be traced in the more homogeneous bodies of such as possess no nucleus. In certain Rhizo- 

 pods belonging to the former category, the body of each individual contains several nuclei, as 

 well as several contractile vesicles ; and thus, as Prof. Schultze has pointed out (xcix, pp. 

 315 — 9), it may be regarded as formed by an aggregation of what would, if still more completely 

 differentiated, have constituted so many distinct cells. In the highest of these composite 

 forms (^ 20), indeed, it seems probable that complete cells may be developed in the peri- 

 pheral portion of the body, whilst its interior retains the condition of homogeneous protoplasm. 



7. In a large proportion of Rhizopods the body has the power of forming upon its surface 

 a firm, testaceous envelope. This, in some instances, appears to be wholly composed of an 

 organic substance resembling chitine ; but in general it is either siliceous, as in the Folycyslina, 

 or calcareous, as in the Foramini/era, the mineral constituent in each case being combined 

 with an organic basis-substance. In certain of the last-named group (^ 60 ) the calcareous 

 constituent is wanting, and the " test" is composed of foreign particles, so regularly arranged and 

 cemented together as to give it the shape and general aspect of one formed on the ordinary plan. 

 The ordinary " tests" of Polycystina and Foraminifera, like the loricce of Diatomacese, must be 

 regarded as generated by a kind of e.xcretion from the surface of the body ; and although the 

 elaborate structure of those of the most highly organized Foraminifera might seem, at first 

 sight, to indicate a less simple origin, yet it will be hereafter shown to be readily explicable 

 according to that viisw (^ 59). There are Rhizopods, on the other hand, in which the body, 

 instead of being invested with a sihcified " test," is strengthened by siliceous spicules difi'used 

 through its substance ; and as there is evidence that in certain cases (^ 18) such spicules are 

 formed by an excretion-process around pseudopodial prolongations of the sarcode, it is pro- 

 bable that this is the general fact, more especially as Mr. Bowerbank has shown (viii,p. 282) 

 tliat the spicules of Sponges have originally a cavity in their interior, which is occupied by 

 soft animal substance. 



8. The subdivision of this Class into Orders seems (in the opinion of the author) to be 

 most satisfactorily accomplished, by taking as a basis those structural characters which are 

 most expressive of physiological differences. Such characters are presented in the form, 



