7G COMPAEATIVE ANATOMY. 



of the organism is, when compared with the cytod stage, the ex- 

 pression of a considerable advance, and prevents us from uniting 

 the forms that do possess it with those that do not, whatever other 

 points of agreement there may be in their protoplasm ; even though 

 it is quite clear that in these cytod forms (Monera) we have the 

 beginnings of the higher stages. These beginnings vary greatly, 

 correspond to separate divisions of more developed forms, and 

 make it probable that the latter have had a polyphyletic origin. 



Of the groups of the Protista I regard the Rhizopoda, Gre- 

 garinas, and Infusoria as Protozoa. 



There is no permanent limitation of the protoplasmic body in 

 the Rhizopoda; their protoplasm sends out varying processes. 

 The lowest division is that of the AmcebidaB, the organism of which 

 is of the grade of a cell. The body is formed by protoplasm and 

 a nucleus ; it is ordinarily naked, but it can temporarily surround 

 itself with a capsule, or the capsule may be a permanent covering, 

 open at one or two points. The organism communicates with the 

 outer world by this opening, and by it can extend itself over its 

 shell. If there are several nuclei present the organism forms a 

 syncytium. The Foraminif era form the second division. It is 

 very probable that they all have a nucleus ; and these organisms, 

 therefore, are also similar to a cell. But the formation of a covering, 

 provided with numerous pores, and often considerably complicated, 

 is an indication of a tendency to higher development. The Heliozoa 

 form a small group more nearly allied to the next division. The 

 Radiol aria, finally, are distinguished from all o titer Rhizopoda by 

 the possession of a "central capsule" within the body. The central 

 capsule contains a number of nuclear structures ; these, indeed, 

 render the Radiolaria similar to cells, but their body cannot be re- 

 garded as a cell. Another course of differentiation seems to have 

 affected them. Further, in some the entire capsular protoplasm 

 contains separate cells, which are regarded by many as structures 

 not belonging to the organism (yellow cells). The development of 

 various kinds of supporting organs gives a peculiar character to the 

 Radiolaria. We can make out a large number of axes in the body, 

 by the aid of these skeletons. 



The second division of Protozoa is formed by the Gregarina). 

 An outer limiting portion surrounding the nucleus, that is a body 

 of the grade of a cell, is wanting in the earliest stages only. They 

 pass, therefore, through the Cytod-stage. In the mature organism 

 an envelope, differentiated from the protoplasm within, can be made 

 ont ; and they give indications of higher differentiations of the 

 subjacent layer of protoplasm. An anterior portion is in many 

 separated by a constriction from the cylindrical or band-like body. 



The Infusoria form the third large division. I do not, as do 

 many, include the Flagellata among them. The body, formed of 

 protoplasm, has its external form defined by the differentiation of a 

 cortical' layer. This cortical layer surrounds the more indifferent 

 protoplasm, which may in many cases be soon to be rotating, and so 



