THE ALGAE 73 



with a smooth or stellate membrane and brownish-red contents (Figs. 53 

 and 54). It germinates to form a single monoploid zoospore, which by 

 division gives rise to a new small colony which undergoes the usual eversion 

 during its development (Fig. 55). 



In the above examples we see the progressive development of the colonial 

 type from the simple, unicellular Chlamydomonas to the highly complex 

 colony of Volvox, with its very many component cells. We also see the gradual 

 division of labour, from the condition in Pandorina in which all the cells of 

 the colony are alike and able to reproduce, through Eudorina, in which there 

 is a slight division of labour, in as much as the cells of the anterior and 

 posterior ends show a difference in their morphology, to Volvox where there 

 are specialized reproductive cells. We also see a tendency towards co- 

 ordination, in the development of protoplasmic connections between the 

 cells of the colony. We do not know the precise function of these connections, 

 but it seems reasonable to suggest that they may in some way assist in con- 

 trolling the movement of the complex colony of Volvox, and it is significant 

 that the connections are more clearly developed in that type than in 

 Eudorina, and that they have not been demonstrated in Pandorina. Another 

 evolutionary tendency which we may notice in this series is the progressive 

 elaboration of the sex organs. In Chlamydomonas the gametes are usually 

 isogamous, in Pandorina they are anisogamous, while in both Eiidorina and 

 Fo/roA' they are clearly differentiated into a small male gamete and a large female 

 gamete which is fertilized without liberation, that is to say, is oogamous. 



This Volvox series, as it is called, is interesting therefore, because by 

 means of a few types we can illustrate apparent stages in an evolutionary 

 series ; not, it is true, one which progressed very far, for it was apparently a 

 side line culminating in Volvox, but one in which the present-day repre- 

 sentatives of the various stages enable us to see the way in which this evolution 

 took place. Its weakness seems to have been the retention of motility by the 

 whole colony, since there are obvious mechanical limitations to progress 

 along this line, and we shall see that all higher plants have evolved on the 

 basis of retaining motility only for the reproductive cells. 



Chaetophorales 



The Chaetophorales are Chlorophyceae in which the thallus is divided 

 into a flat, prostrate system of branched filaments attached to a substratum, 

 and a projecting system of filaments which branch and grow upwards from 

 the prostrate part. Such a thallus is said to be heterotrichous. The upright 

 branches may develop hairs which are either composed of single cells or of 

 rows of narrow elongated cells with scanty colouring matter. In Coleochaete 

 they are merely outgrowths of the walls of the cells of the filaments and 

 may be regarded as setae or bristles. 



Reproduction is very variable. In many forms sexual reproduction is 

 isogamous, with small motile gametes, in others it is oogamous, while in the 



