MICROSCOPIC FORMS OF ANIMAL LIFE. 7 



the bridge-like connections into a different band, it not unfrequently 

 meets a current proceeding in the opposite direction, and is thus carried 

 back to the bod/ without having proceeded very far from it. The pseu- 

 dopodial network along which this 'cyclosis' takes place, is continually 

 undergoing changes in its own arrangement; new filaments being put 

 forth in different directions, sometimes from its margin, sometimes from 

 the midst of its ramifications, whilst others are retracted. Not unfre- 

 quently it happens that to a spot where two or more filaments have met, 

 there is an afflux of the protoplasmic substance that causes it to accumu- 

 late there as a sort of secondary centre, from which a new radiation of 

 filamentous processes takes place. Occasionally the pseudopodia are en- 

 tirely retracted, and all activity ceases; so that the body presents the ap- 

 pearance of an inert lump. But if watched sufficiently long, its activity 

 is resumed; so that it may be presumed to have been previously satiated 

 with food, which is undergoing digestion during its stationary period. 

 No encysting process has been noticed in Lieberkuhnia; and the manner 

 in which this type reproduces itself is at present entirely unknown. As 

 the marine type of it occurs on our own coasts, the fresh- water type may 

 very likely be found in our ponds; and either may be recommended as a 

 most worthy object of careful study. 



RHIZOPODA. 



396. "We now arrive at the group of Rhizopods, or ' root-footed ' 

 animals, first established by Dujardin for the reception of the Amoeba 

 ( 403) and its allies, which had been included by Prof. Ehrenberg 

 among his Infusory Animalcules, but which Dujardin separated from 

 them as being mere particles of sarcode (protoplasm), having neither the 

 definite body-wall nor the special mouth of the true Infusoria, but put- 

 ting forth extensions of their sarcodic substance, which he termed 

 pseudopodia (or false feet), serving alike as instruments of locomotion, 

 and as prehensile organs for obtaining food. According to Dujardin's 

 definition of this group, the Monerozoa already described would be 

 included in it; but it seems on various grounds desirable to limit the 

 term Rhizopoda to those Protozoa in which the presence of a nucleus, 

 the differentiation of an ectosarc (or firmer superficial layer of proto- 

 plasm) from the semi-fluid endosarc, together with the more definite 

 form and restricted size, indicate a distinct approach to the condition of 

 true cells. Many different schemes for the classification of the Rhizopods 

 have been proposed; but none of them can be regarded as entirely satis- 

 factory, our knowledge of the Reproductive processes, and of other 

 important parts of the life-history of these creatures, being still extremely 

 imperfect. And as some parts of the scheme proposed by the Author 

 twenty years ago, 1 based on the characters of the pseud opodial extensions, 

 have been accepted by more recent systematists, he thinks it best still to 

 adhere to it, as seeming to him to be on the whole most natural. 



i. In the First division, Reticularia, the pseudopodia freely ramify 

 and inosculate, so as to form a network, exactly as in Lieberkuhnia; 

 from which they are distinguished by the possession of a nucleus, and by 

 the investment of their sarcodic bodies in a firm envelope. This is most 

 commonly either a calcareous shell of very definite shape, or a test built 

 up of sand-grains or other minute particles more or less firmly united by 



1 "Natural History Review," 1861, p. 456; and "Introduction to the Study of 

 the Foraminifera" (1862), Chap. n. 



