48 MANUAL OF ZOOLOGY. 



Rhizopoda; so that the pseudonavicella in this condition is 

 very similar to an adult Amoeba (fig. i, i). Finally, these 

 amcebiform bodies are developed into adult GregarincB. It 

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

 navicellae 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. 



PsorospermIjE. — There occur as parasites on and within 

 the bodies of fishes certain vesicular, usually caudate, bodies, 

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

 blematical. According to Lieberkiihn they occasionally give 

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

 rated from the pseudonavicellse of Gregarinidm. 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. 

 RHIZOPODA. 



General Characters of the Rhizopoda. — The Rhizopoda 

 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 ari 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 

 sarcode, sometimes very delicate and of considerable length, 

 at other times more like finger-shaped processes; and they 

 are somewhat analogous to the little processes which are oc- 

 casionally thrown out by the white corpuscles of the blood and 

 by pus-cells. Indeed it has been remarked by Huxley that 

 an Amoeba is structurally " a mere colourless blood-corpuscle, 

 leading an independent life." 



