MICROSCOPIC FORMS OF ANIMAL LIFE. 



13 



401. Under the generic name Actinoplirys was formerly ranked the 

 larger but less common Heliozoon now distinguished as Actinosphmrium 

 Eichornii (Fig. 286); one important difference consisting in the struc- 

 ture of the radiating pseudopodia, each of which has here a firm axis-fila- 

 ment or 'spine/ which passing through the superficial zone, rests on the 

 surface of the central sphere, as shown at a a, Fig. 287. This axis is 

 clothed with a layer of soft sarcode derived from the superficial or cor- 

 tical zone of the body. Several nuclei (n, n) are usually to be seen em- 

 bedded in the protoplasmic mass. The general life-history of this type 

 corresponds with that of the preceding; but its mode of reproduction 

 presents some marked peculiarities. The binary segmentation is pre- 

 ceded by a withdrawal of the pseudopodia, even their clearly-defined axis 

 becoming indistinct and finallydisappearing; the body becomes enveloped, 



FIG. 286. 



ActinosphcBrium Eichornii: m, endosarc ; r, ectosarc ; c, c, contractile vacuoles. 



by a clear gelatinous exudation, which forms a kind of cyst; and within 

 this the process of binary subdvision is repeatedly performed, until the 

 original single mass is replaced by a sort of morula ( 391), each spherule 

 of which shows the distinction between the central and cortical regions, 

 the former including a single nucleus, whilst the latter is strengthened 

 by siliceous deposit into a firm investment. After remaining in this 

 state during the winter, the young Actinosplicericv come forth in the 

 spring without this siliceous investment; and gradually grow into the 

 likenesses of their parent. 



402. A large number of new and curious fresh-water forms of this 

 type have been recently brought under notice; of which the Clathrulina 

 elegans (Fig. 288) may be specially mentioned as presenting an obvious 

 transition to the Polycystine type ( 504). This has been found in various 



