CCENOBIEjE 299 



Askenasy has observed the development of the Pediastrum-coenobe 

 by another method from the polyhedra form, previously regarded as. 

 a distinct genus under the name Polyedrium (Nag.). In this form each 

 individual consists of a minute flat angular cell often provided with 

 spines or hook-hke processes. From this a Pediastrum-disc is developed 

 in precisely the same way as from a Pediastrum-cell. The cell-contents 

 break up into a number of megazoospores, which escape in the form of 

 a plate after swarming about for a time ; then, losing their cilia, and 

 placing themselves in a plane side by side, develop into an ordinary 

 Pediastrum. Reinsch, on the other hand (Notarisia, 1888, p. 493), 

 regards Polyedrium as the type of a separate family of Palmellacese. 



Literature. 



Niigeli— Gattungen einzelliger Algen, 1849. 



Braun — Verjiingung in der Natur, 1851 (Ray Soc, Bot. and Phys. Mem., 1853) ; and 



Algee iinicellnlares, 1855. 

 Lagerheim — Bot. Centralbl. , xii. , 1882, p. 33. 

 Askenasy— Ber. Deutsch. Bot. Gesell., 1888, p. 127. 



Order 4. — Pandorine^. 



In the higher genera of this order, Pandorina (Ehrb.), Gonium (Miill.), 

 and Stephanosphsera (Cohn), the individual is a spherical or tabular 

 coenobe, the cells of which are united together by a gelatinous matrix 

 with a definite bounding-wall. With them are associated also some 

 unicellular organisms, Chlamydococcus (A. Br.) and Chlamydomonas 

 (Ehrb.), which may possibly be connected with them by a process of 

 degeneration. Whether isolated or associated, each cell possesses a 

 pair of whip-like vibratile cilia attached to the anterior pointed end, by 

 means of which it is rapidly propelled through the water ; in the case 

 of the social genera these cilia project through the common gelatinous 

 envelope of the colony. Multiplication takes place either non-sexually 

 by simple subdivision of the cells of a colony, or sexually by the union 

 of two (or occasionally more than two) zoogametes into a resting zygo- 

 sperm. A characteristic feature of the family is the formation of a 

 colony of cells within each cell in the mother-colony. The organisms 

 here included were described by Ehrenberg as constituting a family of 

 Infusorial Animalcules. They live, associated with larger algae, in fresh 

 water, running or stagnant, often in such quantities as to impart to it a 

 green colour. The family closely approaches Volvocineae through 

 Eudorina. 



Of the unicellular Pandorines Chlamydomonas Ehrb. may be taken 

 as a type. In the form in which it is known under this name, it 



