THE MASTIGOPHORA 181 



ORDER 2. Volvocina. 



Individuals biflagellate, arranged in colonies called " coenobia," 

 of definite forms, with a gelatinous matrix. Reproduction takes 

 place by the cleavage of certain individuals (cells) of the colony 

 called the gonidia. There are two kinds of gonidia the partheno- 

 gonidia or asexual forms, and the gametogonidia or sexual forms. 

 The gametogonidia consist of the oogonidia or female gametes and 

 the antherogonidia or spermatozooids. These conjugate to form 

 the zygotes. 



The volvocine colony is physiologically an individual organism, 

 exhibiting histological differentiation and correlated locomotor 

 activities of the constituent cells. In Eudorina the cells are 

 differentiated into male and female, the male cells arising from the 

 anterior quartet, the remainder becoming female. In Volvox the 

 reproductive cells, both parthenogonidia and gametogonidia, are 

 limited to a few of the cells which compose the coenobium. In 

 Pleodorina the parthenogonidia are confined to the posterior 

 hemisphere (Fig. 9). 



Protoplasmic intercellular connections between the cells (in- 

 dividuals composing the colony) only occur in the genus Volvox, 

 in apparent correlation with the high degree of individuation 

 attained by this form. Each cell or " coenocyte " is contained 

 within its own capsule, which is separated from neighbouring 

 capsules by a radial cell-wall. The sarcode is separated from the 

 cell-walls by a wide space which is occupied by the gelatinous 

 matrix, and protoplasmic processes radiate through the matrix and 

 traverse the cell-walls (Fig. 5 (18)). 



The coenobium of Volvox is a sphere c Consisting of a single layer 

 of cells surrounding a central cavity, and thus presents a superficial 

 analogy to the blastula- stage in the embryonic development of 

 Metazoa. The presence of flagella, eye -spots, and contractile 

 vacuoles attest its animal properties, while the presence of 

 chromatophores, pyrenoids, and starch granules proclaim . its 

 vegetable affinities. 



The sphere comprises two differently constituted hemispheres. 

 The trophic hemisphere is that which is directed forwards during 

 locomotion, and the component cells are distinguished by the 

 brighter development of the eye- spots. The other hemisphere is 

 the generative hemisphere, in which the obgonidia, antherogonidia, 

 and parthenogonidia are chiefly formed. 



Locomotion is rotatory, i.e. forward progression accompanied by 

 rotation about the main axis either to the right or to the left, 

 though sinistral rotation is more frequent than dextral. 



In Volvox globator, L., the average number of cells in a mature 

 coenobium is 10,000, the actual numbers ranging from a minimum 



