CRYPTOGAMS 



321 



are also provided in their vegetative state with cilia and surrounded hy a delicate 

 envelope. The cilia, usually two in number, project through this external envelope, 

 and by means of them the Algae of this family are enabled to swim freely about. 

 In this respect they continue their vegetative existence in that condition which, in 

 the case of the other Protoeoccoideae, is only assumed transitionally by the swarm- 

 spores. The multiplication of the Volwcaceae is effected by simple division of the 

 ciliated cells ; their sexual reproduction by conjugating gametes or by means of egg- 



Fig. 242. — Volvox aureits. A, Colony with three eggs, o, shortly after fertilisation ; a, spermato- 

 zoid-packets in process of development; t, vegetative daughter colonies (X180);B, sper- 

 matozoid- packet of 32 cells, seen from above; C, the same seen from the side (x 68?); -D, 

 spermatozoids ( x S24). (After L. Klein.) 



cells fertilised by spermatozoids. The genus Sphaerella (ffaematococcus) belongs 

 to the simplest solitary forms of this family, the presence of some forms of which 

 (particularly <S'. pluvialis), on account of the hsematochrome contained in their 

 protoplasm, often impart a bright red colour to small pools of water in which they 

 are found. Sphaerella nivalis, another species of this same genus, is also the cause 

 of the so-called "red-snow " of the snowfields in high northern latitudes and in the 

 Alps. The swarm-cells have a widely-distended envelope and two cilia (Fig. 241, 

 A). They can withdraw their cilia and become resting - cells, which eventually 

 separate again into several swarm-cells by the division of their protoplasmic contents 



Y 



