MESOZOA. 61 



Protozoa Flagellata, among which it is placed by 

 Biitschli, who, nevertheless, says that strictly speaking 

 the genus does not belong here, since the so-called colo- 

 nies are in reality many-celled individuals. 1 



We have in Volvox an organism of many cells which 

 are arranged in one layer around a central cavity. This 

 cavity is hollow in so far as it is destitute of cells, though 

 it is filled with a gelatinous cellulose substance which is 

 secreted by the cells and in the periphery of which they 

 lie embedded, connected by delicate threads of proto- 

 plasm. 



It has already been shown that many adult Protozoa 

 represent the simple unfertilized egg, and that probably 

 some of the most specialized members of this branch, 

 such as Vorticella, reach in their development the condi- 

 tion of the fertilized egg. If, now> this egg were to 

 divide and the products of division remain together and 

 arrange themselves in a layer around a central cavity, 

 then we should have the next stage of development of 

 the fertilized egg, known as the blastula, which is well 

 represented by the adult Volvox. 



In Volvox the peripheral layer is made of flagellate 

 motor and feeding cells called somatic cells (PL 59, fig. 

 i). There may be 12,000 of these cells in an adult 

 and each one has two long whips which pierce the outer 

 wall surrounding the cells. A few of these somatic cells 

 which migrate from the surface and are just inside of this 

 peripheral layer have the power of dividing or of asexual 

 reproduction, and are known as parthenogonidia (PI. 59, 

 fig. 2 a-e, illustrating the process of division) . These 

 parthenogonidia give rise to asexual adults (fig. 3) that 

 often contain eight smaller Volvoces which revolve within 

 the parent, and each of these in turn contains eight more, 

 so that three generations are represented as seen in fig. 

 3. When the parent capsule is ruptured, the eight smaller 



1 Bronn's Thierreich, II, 1883, p. 775. 



