THE SARCODINA 245 



ment (compare p. 90). In the first or Actinophrys-type (Fig. 46) 

 the nucleus is central, and the pseudopodia are centred on it. 

 Actinosphcerium (Fig. 3) can be derived from this type by multi- 

 plication of the nucleus, originally single, until there may be some 

 hundreds present in large specimens. The marine form Campto- 

 nema nutans, Schaudinn, is perhaps also to be referred to this type 

 of structure ; it has as many pseudopodia as there are nuclei present, 

 each pseudopodium arising directly from a nucleus (p. 91, Fig. 47). 



In the second or Acanthocystis-type (Figs. 18, 64) the centre of 

 the spherical body is occupied by a " central grain " (p. 91), on 

 which the axial rays of the pseudopodia are centred. The nucleus, 

 on the other hand, occupies an excentric position in the body. In 

 this type there is a tendency to a sessile habit of life, the animal 

 being attached by the surface of the body, which may grow out 

 into a stalk, as in Clathrulina (Fig. 19). In the interesting marine 

 genus Wagneretta (Fig. 48), the surface of attachment has become 

 drawn out in such a way that the body is divided into three parts 

 basal plate, stalk, and head. The nucleus is situated in the basal 

 plate. The head contains the central grain, from which the pseudo- 

 podia radiate. Thus, in this genus the excentric position of the 

 nucleus is carried to an extreme ; it may be regarded as having 

 grown out from the body in a lobe or prolongation which forms the 

 basal plate and stalk, while the original body remains as the head 

 with the central grain and pseudopodia. 



The skeleton, when present, may take various forms. It may 

 be a simple gelatinous investment, or may contain mineral (sili- 

 ceous) substance either in the form of loose, radiating spicules, as 

 in Acanthocystis, or of a continuous lattice-like investment, as in 

 Clathrulina. In Wagner ella the basal plate and stalk are protected 

 by a tough yellowish organic membrane, replaced in the head by 

 a colourless gelatinous layer, and both head and stalk are further 

 protected by siliceous spicules, which are formed in the protoplasm 

 and transported by protoplasmic currents (Zuelzer, 86). 



Life- History. Reproduction in the free vegetative phase is 

 effected by binary fission or gemmation. Imperfect binary fission may 

 lead to colony-formation, as in Rhaphidiophrys. The sexual phases 

 are only known accurately in a few cases. In Actinophrys, Schaudinn 

 described copulation within a cyst (p. 132, Fig. 71), with subse- 

 quent division of the zygote and liberation of two individuals from 

 the cyst. In Actinosphcerium (Hertwig), encystment of a large 

 multinucleate individual is followed by degeneration of about 

 95 per cent, of the nuclei ; the remainder appear to fuse in pairs, 

 and the body then divides into aa many cells as there are nuclei. 

 Round each cell a separate " prinfary " cyst is secreted within the 

 gelatinous " mother-cyst " originally formed round the whole mass. 



