162 Volvoeinese 



higher types, there is marked heterogamy, sexual reproduction occurring 

 by the union of heterogametes. Various grades of heterogamy can be ob- 

 served in the Volvocine series, the highest types exhibiting true oogamy 

 with highly specialized antherozoids and oospheres. The zygote invariably 

 rests for a period before germination. 



In passing from the lower to the higher members of the Volvocineae 

 there is a more striking progressive evolution of forms than is exhibited 

 in any other family of Alga3 ; indeed, it is one of the most perfect evolutionary 

 series known to the biologist. There is a progressive increase in the number 

 of cells and in the size of the cuenobium, accompanied by a marked differenti- 

 ation of reproductive from vegetative (or somatic) cells, and associated with 

 these phenomena is a gradual replacement of isogamy by heterogamy, ulti- 

 mately reaching the highest condition of oogamy. 



The Volvocinea? connect the Green Algre with the Flagellata, an extensive 

 group of unicellular organisms of very varied character from which it is now 

 generally recognized that the Chlorophycere have been phylogenetically 

 derived. Klebs ('92) was the first investigator to study in a careful and com- 

 prehensive manner the varied organisms included in the Flagellata, which 

 are a group of the Protista exhibiting a mixture of animal and vegetable 

 characters. Since then our knowledge of the Flagellates has greatly in- 

 creased, and it is well known that among the heterogeneous assemblage of 

 forms embraced in the group the distinction between animal and vegetable 

 organisms entirely breaks down. In the Flagellata are found the phylo- 

 genetic starting-points of many lines of descent, both animal and vegetable, 

 and the group is one which should be studied by every serious student of 

 biology. Klebs drew up the characters which distinguish the Flagellata 

 from the Volvocineae, and he showed that a great consensus of characters 

 united the Volvocinese with the Green Alg;e and separated them from the 

 organisms included in the Flagellata. All subsequent researches have 

 consistently supported Klebs' statements, and the Volvocinege must therefore 

 be regarded as a group of motile Green Algre 1 . 



Oltnianns ('04) and Pascher ('12) have both placed the Volvocineee as a group the 

 'Yolvocales' equivalent in rank to the Protococcales or Ulotrichales. It must be 

 confessed, however, that a careful enquiry into the general organization of the Volvocine* 

 scarcely supports this view, which gives undue prominence to the unicellular and colonial 

 motile forms of the Isokontte. These certainly constitute a distinct group, not to be 

 confused with any other ; and indeed, the beautiful evolutionary series of the Volvocaceee is 



1 The inclusion of the Volvocineae in a modern text-book of Zoology is difficult to reconcile 

 with the present state of our knowledge of the Green Algas and the Flagellata, and can only be 

 regarded as a relic of the chaos which existed a quarter of a century ago. 



Dangeard's work ('98; '01) on the nuclei of the Chlamydomonadese has also demonstrated 

 that in contrast to the primitive and divergent types of nuclear structure and nuclear division 

 exhibited by the Protozoa, the mitosis, even of the lowest of the Volvocacea, does not differ 

 essentially from that which occurs in higher plants. 



