170 



Volvocineae 



Fig. 99. Spondylomorum qun- 

 ternarium Ebrenb., a colony 

 of 16 cells, n, nucleus ; s, 

 stigma; v, contractile vacu- 

 ole. (After Stein, from 

 Wille.) x 650. 



fuse in pairs to form zygotes (fig. 98 D F). One species of Carteria occurs 

 symbiotically in the marine Planarian worm Convoluta roscoffensis (Keeble & 



Gamble, '07); this species is also of interest on 

 account of the very few marine species of the Volvo- 

 cacea> which are known to exist. An interesting 

 genus is Scherffelia Pascher ('12), in which the cell 

 is compressed, so that in cross-section it has some- 

 what the appearance of a thin biconvex lens. There 

 are two plate-like chloroplasts, sometimes united 

 posteriorly in Scherffelia Phacus, and no pyrenoid is 

 present. In Sch. Phacus the cell-wall is laterally 

 expanded into the rudiments of wings such as are 

 more prominently developed in Pteromonas. 



Tetrablepharis (fig. 102 G) is a colourless genus 

 which is probably a degenerate derivative of Carteria, 

 having assumed a saprophytic mode of existence. 



The only colonial type is Spondylomorum qna- 

 ternarium Ehrenb. which although known from both 



the New and the Old Worlds is much the rarest ccenobic form of the Volvo- 

 cacea?. The disposition of the cells forming the ccenobium is unique amongst 

 the Volvocinea?. There are sixteen cells arranged in four tiers of four cells 

 each, those of any one tier alternating with those immediately in front or 

 behind. The cells are arranged on a central gelatinous axis so that their 

 anterior extremities are directed approximately to the front end of the colony 

 (fig- 99). 



The genera are : Carteria Diesing, 1868 [inclus. Pithixnis Dangeard, 1888, and Corbiera 

 Dangeard, 1888] ; Scherffelia Pascher, 1912 ; Spondi/lomorum Ehrenberg, 1848 ; and the 

 colourless Tetrablepharis Semi. 



Sub-family CHLAMYDOMONADK^E. The most important genus of this 

 sub-family is the unicellular Chlamydomonas. The cells are spherical, ovoid, 

 subcylindrical, or rarely somewhat fusiform in shape, provided with a thin 

 cell-wall and with two cilia. The chloroplast is very variable in form, but it 

 is typically cup-shaped and occupies the posterior region of the cell, more or 

 less surrounding a central part of the protoplast in which the nucleus is 

 lodged (fig. 98 H and /). The genus includes approximately 30 species, in 

 most of which there is a single pyrenoid in the chloroplast ; but in some there 

 are several pyrenoids and in others none. There is usually a pigment-spot 

 in a lateral position, generally anterior but sometimes posterior. Each cell 

 possesses a distinct cell-wall which sometimes gives a cellulose reaction and 

 sometimes does not ; and there is evidence to show that in the older individuals 

 the cellulose reaction is entirely obliterated owing to the impregnation of the 



