164 Volvocinese 



must be regarded as closely related. For these two genera he proposed 

 a new family, the Sphaerellaceae. This proposition has been further sup- 

 ported by the researches of Wollenweber ('08). A new classification of the 

 'Volvocales' has recently been set forth by Pascher ('12), in which he accepts 

 the family Sphserellaceae and proposes another, the Carteriacese, to include 

 those members of the Volvocinese which are furnished with four cilia. The 

 three families of the Carteriaceae, Spha?rellacese and Chlamydomonadacese, 

 as outlined by Pascher, are open to criticism since they do not appear to 

 be parallel groups. The Sphasrellacese are primitive with very distinctive 

 cytological characters, but the Carteriacese and Chlamydomonadaceae agree 

 absolutely in the main points of their cytological structure except for the 

 four cilia of the former and the two possessed by the latter. Such a 

 character can hardly be accepted as the sole basis of separation of two 

 families of Chlamydomonads when zoogonidia (which are themselves homo- 

 logous with Chlamydomonadine cells) possessing sometimes two and sometimes 

 four cilia are found in the same species of certain of the Ulotrichales 1 . 



The diagram on p. 163 contains the suggestions for a possible scheme 

 of evolution of the Volvocinese. It will be seen that the Volvocacese are 

 derived from the Polyblepharidacese, but that the Sphserellacese probably 

 originated from some group of Flagellates closely allied to those from which 

 the Polyblepharids were themselves evolved. 



^1 Family Polyblepharidacese. 



This is a small but interesting family the members of which exhibit 

 a mixture of Flagellate and Volvocine characters. They are distinguished 

 from all the other Volvocinese by the absence of a definite secreted cell-wall. 

 The body of the cell is invested only by a protoplasmic membrane (fig. 95 E] 

 and, with the possible exception of Pyramimonas, is able within prescribed 

 limits to undergo certain changes of form. Two, four, or more cilia (often 

 termed flagella) are attached to the anterior end of the cell. These cilia 

 are rather thick (much thicker than those in the Volvocacese), and in Pyra- 

 mimonas delicatulus Griffiths ('09) a weak solution of commercial formalin 

 blisters them in such a way (fig. 95 H} as to indicate that the cilium is not 

 a homogeneous rod, but a structure in which the peripheral part is of a 

 denser character than the more central portion. The movements of some 

 species have been stated to be rather more of the Flagellate than of the 

 Volvocine type, but this is not so in Pyramimonas and Dunaliella. The 

 chloroplast is of the typical Volvocine kind and contains a pyrenoid. A 

 pigment-spot is present in some forms but absent in others. 



1 Pascher has attempted to overcome this difficulty by splitting up the genera Ulothrix and 

 Stigeocloniuin ( = Myxonema) on grounds which appear to be inadequate. 



