OF THE PHYTOZOA. 181 



cli^•ision. The result of these considerations led Ehrenherg to perceive the 

 true homology between the spheres of Volvocc and the four-sided tablets of 

 Gonium. 



Each member of the colony, he added, has an individuality of its own, and 

 to all appearance resembles an ordinaiy simple monad, enclosed mtliin a 

 lorica (a Jaeerna), having a red eye-speck, a double filiform proboscis or 

 filament protruding from the suiface of the common spherical lorica and 

 giving the hairy or ciliated appearance to it, and at the base of these filaments 

 a mouth, indicated by a bright, clear spot. Internally Ehrenherg believed 

 he discerned clear digestive cells, a contractile seminal vesicle, one or two 

 roimd sexual glands, and numerous green ova. The following histoiy of 

 Volvox conveys the present state of information and opinion on this interest- 

 ing organism. 



The globes of Volvox are bounded externally by a hyaline structui-eless 

 membrane or pellicle, which corresponds to the " envelope-cell" as understood 

 by Cohn (XX. 34, 45). Distributed on the inner wall or surface of this mem- 

 brane, is (in Cohn's words /. M. S. 1857, p. 140) " an infinitude of very 

 minute hexagonal cells, attached to each other in the same way as are the 

 elements of an epidermic tissue" (XX. 38). The protoplasmic matter or 

 the endochrome of each cell constitutes the presumed monad of Ehi-enberg, 

 which is flask-shaped, and protrudes its tapering extremity or neck outwards, 

 bearing at its apex two ciliaiy filaments which penetrate the common envelope 

 and vibrate freely in the surrounding water. 



The green substance composing each monadiform individual or "^ primordial 

 cell," is the usual vegetable protoplasm, and contains chlorophyll- vesicles, a 

 clear globule or nucleus, one or rarely two contractile vesicles, and usually a 

 brownish-red speck, regarded by Ehrenherg as a ^dsual organ (XX. 35) ; the 

 filaments are, as usual, productions from the protoplasm. Further, each green 

 globule is enveloped in one or more partially organized special membranes, 

 which are in more or less close apposition with it according to the age and 

 the conditions of Kfe under which the Volvox is placed (XX. 35, 37), and give 

 it the essential characters of a cell vrith a cell- wall. In the early stage of 

 development the several protoplasmic masses in a colony are closely aggre- 

 gated ; but as age advances, a clear interval surrounds and separates them, 

 traversed by several prolongations of the protoplasmic matter connecting 

 together adjoining cells (XX. 37). These processes extend outwards hke so 

 many rays from each primordial cell, and as a rule encounter those fi^om sur- 

 rounding cells at a determinate distance, where they meet with an external, 

 deHcate, transparent membrane — the wall of the cell of which the central 

 green globule represents the nucleus. This thin membrane forms the boimd- 

 ary of each clear space surrounding the contained green globule ; and from 

 the mutual pressure of the assemblage of cells composing the Volvox, it 

 acquires, as seen from above, a hexagonal figiu'e (XX. 38, 39-41, 45). 



We have observed abeady that, as age advances, the space or areola around 

 each primordial ceU increases ; that is, the external cell- wall becomes further 

 detached from the contained protoplasmic mass, and hence the processes con- 

 necting the two — at first, and even for some considerable time during active 

 nutrition, thick, clumsy, and irregular (XX. 42, 43) — become gradually 

 stretched, until they are eventually converted into attenuated threads or 

 almost imperceptible lines. In fact, by over-distension of the cells from 

 any cause, whether, as commonly happens, from advancing age, or from the 

 breaking up of the globe, these cords get mptui-ed, and then, by retracting 

 themselves from the outer delicate ceU-waU, coalesce with the protoplasmic 

 central mass. On comparing this structm-e mth that of Goniwn as recorded 



