504 Annals of the South African Museum. 



and Plate XXXVII, B), in which no limiting membranes can be seen. 

 Even the gelatinous matrix is invisible until stained. With methy- 

 lene blue it turns purple, and it is clear that there is no differentiation 

 of a surface membrane. Apparently this botryoidal form only 

 arises from very young colonies, since the spheres and their con- 

 stituent cells are always very small and delicate. This form was the 

 rarest, and occurred most abundantly in material which contained 

 fairly advanced stages of Volvox gigas. 



Finally, yet a third form, the Tetraspora-\ike form (fig. 6, E), 

 occurs at first in small quantities (Plate XXV, D, top left-hand 

 corner), but rapidly increases until it forms nearly the whole of the 

 phytoplankton (Plate XXXVI, E, and Plate XXXVII, E). Appar- 

 ently it may arise either from the botryoidal form or from the 

 spherical. In the former case the constituent cells divide repeatedly, 

 the gelatinous matrix extends and its composition alters slightly, 

 the outer layers becoming denser. Transition stages from one form 

 to the other are occasionally seen, though they are not common. 

 When a colony of the spherical type is passing over into the Tetraspora- 

 type (fig. 6, D, and Plate XXXVII, D, E), the cells of the groups 

 divide as if to form daughter colonies, but the resultant cells continue 

 to divide more or less irregularly (Plate XXXVII, D), some at any 

 rate forming biciliate zoospores which move through the gelatinous 

 matrix ; this has meanwhile been losing its firm outline, often 

 becoming irregularly lobed (fig. 6, D). Eventually all trace of 

 grouping of the cells is lost, and an irregular gelatinous mass, packed 

 with small rounded cells (Plate XXXVII, C) similar in structure to, 

 but smaller than, those of the spherical form of the alga, is formed. 



There is no trace of pseudocilia, and here, as in the spherical form, 

 zoospores of two sizes are often seen, some ranging from 4 x 7 fj. to 

 5 x 9 /x, others from 8 x 10 yu, to 9 or 10 x 11 /LI ; the cilia are equal in 

 length to, or slightly longer than, the protoplast, the chloroplast 

 cup-shaped with nearly central pyrenoid. The zoospore is elongated, 

 slightly oblique, with a very small apical protuberance and central 

 pyrenoid (fig. 6, H). 



Thus the characteristic Tetraspora-\ike palmelloid stage consists 

 of irregularly shaped gelatinous masses containing very large numbers 

 of rounded cells without ciliary appendages of any form, except 

 where the round cells have divided to form small elongated bi-ciliate 

 zoospores. The colonies may be more or less rounded, but are more 

 often elongated, irregularly lobed or branched, often 2 mm. or more 

 in length. The cells, similar in form and structure to those of the 



