42 MANUAL OF ZOOLOGY. 



they are termed ' pseudonavicellae ' (or ' pseudonaviculee ') 

 (fig. 1 c). The next step in the process consists in the libera- 

 tion of the pseudonavicellaa which escape by the rupture of 

 the enclosing cyst (fig. 1 d). If they now find a congenial 

 habitat, they give origin to little albuminous or sarcodic 

 masses, which exhibit lively movements, and are endowed 

 with the power of throwing out and retracting little processes 

 of the body which closely resemble the ' pseudopodia ' of the 

 Bhizopoda ; so that the pseudonavicella in this condition is 

 very similar to an adult Amoeba (fig. 1 e). Finally these 

 amoebiform bodies are developed into adult Gregarince. It 

 will be seen from the above that the formation of the pseudo- 

 navicellge out of the granular contents of the body, subsequent 

 to the disappearance of the nucleus, presents some analogy to 

 the segmentation of the impregnated ovum which follows 

 upon the dissolution of the germinal vesicle. 



PSOROSPERMLE. There occur as parasites on and within the 

 bodies of fishes certain vesicular, usually caudate, bodies, 

 termed Psorospermice, the exact nature of which is very pro- 

 blematical. According to Lieberkiihn they occasionally give 

 origin to amoebiform bodies, similar to those which are libe- 

 rated from the pseudonavicellse of Gregarinidce. In this case 

 they should probably be regarded as the embryonic forms 

 of some Gregarina. By Balbiani, however, they are looked 

 upon as properly belonging to the vegetable kingdom. 



CHAPTER II. 

 SHIZOPODA. 



GENERAL CHARACTERS OF THE RHIZOPODA. The Eliizopoda 

 may be defined as Protozoa, which are destitute of a mouth, are 

 simple or compound, and possess the power of emitting ' pseudo- 

 podia' They are mostly small, but some of the composite 

 forms, such as the sponges, may attain a very considerable 

 size. Structurally a typical Rhizopod as an Amoeba is 

 composed of almost structureless sarcode, without any organs 

 appropriated to the function of digestion, and possessing the 

 power of throwing out processes of its substance so as to con- 

 stitute adventitious limbs. These are termed ' pseudopodia,' 

 or false feet, and are usually protrusible at will from different 

 parts of the body, into the substance of which they again 

 melt when they are retracted. They are merely filaments of 



