PROTOZOA. 21 



In the naked and more simple form (PI. 8, figs, i, 2 

 probably a variety of the same species) there is, however, 

 no such differentiation of the outer part; fig. i shows 

 the brown food material in the interior and the pseudo- 

 podia, and fig. 2 is a specimen colored and showing 

 probably one stage of division in which the organism is 

 separating into two parts. The nuclear grains are seen 

 in this figure, as also in fig. 5. 



It is reasonable to suppose that a transitional organism 

 exists, or has existed, in which the young stage has nu- 

 clear grains and the adult a well formed nucleus, but we' 

 have seen no such species described or figured. 



SARCODINA. RHIZOPODA. 



The probable intermediate forms just mentioned lead 

 naturally to the group, Amoebina, represented by the 

 Amoeba protens Leidy (PI. 9, figs. i-n). Here we have 

 a typical Rhizopod with the organs and functions peculiar 

 to such an animal. PL 9, fig. i, is probably the young 

 of this species and fig. 2 presumably an older stage. In 

 both the young and the adult (fig. 3) the protoplasm has 

 become more or less differentiated into a clear outer layer, 

 the ectosarc, and an inner granular portion, the endosarc. 

 When, however, one observes by the aid of a microscope 

 the granular endosarc flowing into the clear ectosarc and, 

 as it were, taking possession of it, one becomes convinced 

 that there is no constant line of demarcation between the 

 two. 1 



1 According to Leidy (Fresh-water Rhizopods of North America, 

 U. S. Geol. Surv. Terr., XII, 1879, p. 24}, Dr. Wallich states that 

 the ectosarc is due to a temporary and partial coagulation of the en- 

 dosarc coming in contact with the water in which the animal lives, 

 and it again reverts to the mass of the endosarc within the body. 

 The process reminds one of the cooling of a molten mass of metal 

 at the sides of a crucible, and the melting away again of the crust as 

 it is stirred from the sides into the remainder of the molten mass 

 within. 



