PLATE IX.— THE ROLLING SPHERE (Volvox globator). 



(From DwM-Port.) 



Volvox, so named from its rolling motion, is found in fresh-water pools, and attains a size sufficient to be distinguished by the 

 naked eye. It is a hollow sphere, and the entire periphery is formed of small cells, each furnished with two cilia. Its slow, stately, 

 rolling motion is due to the harmonious action of these cilia. The sphere consists of vegetative cells and reproductive cells. It is so 

 large that it would be a physical impossibility for every cell to undergo a process of division, and as the organization becomes more 

 complex there arises a necessity for a division of labour. The work, which in a simpler form every portion of the organism was fitted 

 to do, has now to be distributed and assigned to certain cells. Cells are thus set apart for the work of reproduction from a very early 

 period, and are much larger than the vegetative cells. Not only so, but the male and female elements are decidedly different ; in the 

 one case being a tapering portion of protoplasm provided with cilia and motile, in the other a stationary rounded ball of protoplasm. 

 Thus the two elements of reproduction are becoming more and more distinct. At first they were undistinguishable, as in Mucor; 

 next distinguishable only in size, as in Pandorina; but now their form as well as their dimensions is different. 



When Volvox is kept in a warm room, it has been observed in some cases that the protoplasm strays from the cells and creeps 

 about in the water after the manner of an amoeba. Here the green protoplasm of a plant behaves like the protoplasm of an animal, 

 putting forth processes and progressing by reason of the contractility of the protoplasm. It shows that the fundamental difference 

 between the lower plants and the lower animals consists in the one being free to move and the other not. Plants have their proto- 

 plasm inclosed in a rigid cell-wall, which curbs and restrains them, and under such conditions the protoplasm is forced to behave 

 differently. 



Fig". 1. Volvox-sphere in the sexual stage. 



Reproductive cells. — Female Zoospores are flask-shaped at first, but finally become spherical. This stationary rounded mass of 

 protoplasm is now called the Oosphere, and with its gelatinous cell-wall is called the Oogonium. 

 Antheridia, containing bundles of Antherozoids, or sperm-cells. 



Fig. 2. Portion of periphery much magnified. 



Reproductive cell relatively large, with nucleus and nucleolus. Vegetative cells smaller, often with red "eye- 

 spot" 

 Fig. 3. The Antherozoids have bored through the gelatinous investment of the Oogonium, and now surround the Oosphere. 

 Fig. 4. The outer investment of the unripe Oospore is a firm and spinous Exosporium, while the inner is a gelatinous Endo- 



sporium. 

 Fig. 5. Antheridium, with its gelatinous investment containing the bundle of Antherozoids. 



Fig. 6. Iodine kills the Antherozoids, and makes their cilia distinct. 



The Antherozoids are of a whip-lash shape, with a pair of cilia towards the rounded end. 



Fig. 7. The Antherozoids have a wriggling movement, caused by the expansion and contraction of their bodies, aided by the 

 cilia. 



