LESS, xxvi PANDORINA 267 



Thjre are several interesting organisms which help to 

 bridge this gulf. Two of the more accessible and well- 

 known forms will now be described. 



Pandorina (Fig. 64, A) is a colony consisting of sixteen 

 zooids closely packed in a gelatinous case of a globular 

 form. Each zooid resembles in general characters a mo- 

 tile Hasmatococcus or Euglena, having an ovoid cell-body 

 coloured green by chlorophyll, a red pigment spot, and 

 two flagella, which protrude through the gelatinous wall of 

 the colony, and by their action impart to it a rotatory 

 movement. 



In asexual reproduction each of the sixteen zooids divides 

 and re-divides, forming at last a group of sixteen cells. In 

 this way sixteen daughter colonies are produced within the 

 gelatinous envelope of the original mother colony (B). By 

 the solution of the envelope the daughter colonies are set 

 free, and each begins an independent existence. 



In sexual reproduction the zooids are set free singly from 

 the colony (c). They swim about actively, approach one 

 another in pairs, and conjugate (D), becoming completely 

 fused together (E) to form a zygote (F). This increases in 

 size and develops a thick cell wall (G). After a period of 

 rest, the protoplasm escapes from the cell ^vall (H), puts out 

 a pair of flagella (i), and swims about. Finally it settles 

 down, divides and re-divides, and so gives rise to a new 

 colony (K). 



It is obvious that Pandorina resembles the polyplast 

 stage of an embryo : moreover it is produced by the repeated 

 fission of a flagellula, just as the polyplast is formed by the 

 repeated fission of an oosperm. 



The beautiful Volvox (Figs. 65 and 66), one of the favourite 

 studies of microscopists, is a colony of Haematococcus-like 

 zooids arranged in the form of a hollow sphere containing a 



