THALLOPHYTA. 631 



pigment hsematochrome, which may almost completely mask its green colour. It 

 was from this circumstance that it received its name of Hoematococcus pluvialis. 

 Hseinatochrome is even more constantly present in SphoBrella nivalis, the "red snow", 

 whose appearance and life-history have already been described (vol. i. p. 39). S. 

 Biitschlii has its hsematochrome concentrated in an eye-spot like that of Chlamy- 

 domonas. 



The VolvocecB differ from the ChlaTnydomonadece in consisting of motile colonies 

 of cells, the members of each colony being united in a common investment. The 

 bodies of the individuals composing the colony are also joined in some genera by 

 protoplasmic processes. The body of each individual is identical, in the fundamen- 

 tal points of its structure, with that of a Ghlamydomonas or of a Sphcerella. The 

 VolvoceoB present us with an interesting series of forms, showing a gradually increas- 

 ing sexual differentiation of gametes, and, in the higher forms, an interesting sub- 

 ordination of the individual to the colony as a whole. 



Gonium is a form in which the colony consists of (usually) sixteen Ghlamydo- 

 monas-like cells arranged in a flat plate, which swims in a line at right angles to 

 its surface, the flagella of the central cells of the disc projecting forwards, those of 

 the peripheral cells obliquely outwards and forwards. All the cells are inclosed in 

 a general mucilaginous envelope, and are joined to one another by protoplasmic 

 processes. 



Reproduction is effected by the division of the constituent cells of the colony in 

 two planes at right angles to one another and to the plane of the colony, so that each 

 mother colony produces sixteen daughter colonies, whose discs of cells all lie in the 

 same plane. Meanwhile, the mother cells are separated from one another by the 

 gradual liquefaction of the general mucilaginous envelope, and thus the daughter 

 colonies become independent. 



Formation of isogametes also takes place, but is not thoroughly understood. 



Stephcmosphcera is a very beautiful form, occurring especially in pools of rain- 

 water collected in rock hollows in hilly districts. It is often found in company with 

 Sphcerella pluvialis. The colony consists of a ring of (usually) eight Sphcerella-\i]s.e 

 cells arranged in the equatorial plane of a spherical or ovoid cellulose membrane. 



When reproduction is about to occur, the constituent cells draw in the protoplas- 

 mic processes by which they are attached to the general membrane; each secretes a 

 membrane of its own, and then its protoplasm divides in two planes to form eight 

 (sometimes seven) daughter-cells. When these have acquired flagella they begin to 

 swarm, and eventually escape by bursting the membrane of the mother colony. 



Gametes are formed in the same way, but usually by more divisions, as many as 

 thirty-two being sometimes produced from a single cell. In most cases all the cells 

 of a colony divide at once to form gametes, but this is not invariably the case. 

 Each bundle of gametes produced from a single cell breaks up, and all the gametes 

 begin to swarm within the colony. The gametes are spindle-shaped, each with two 

 flagella and an eye-spot. They conjugate in pairs, usually inside the general mem- 

 brane, but conjugation never takes place between two gametes derived from the 



