292 



ORDER i. VOLVOCINE^E. 



The well-known Volvo x glob ator L. may be taken as a type of this 

 interesting family. This organism is not uncommon in clear pools, and 

 is visible to the naked eye as a minute pals-green globule, about ^ 

 inch in diameter, rolling through the water, the motion being due to 

 numerous colourless cilia projecting from its surfa'ce. It is one of the 

 most beautiful objects that can be observed under the microscope. 



Under a sufficiently high power of the microscope, Volvox is seen to 

 be a membranous transparent hollow sac studded with green points ; 

 in the interior are darker green globules, the original number of which 

 is apparently always eight. The green peripheral corpuscles or swarm- 

 cells are each provided with a pair of vibratile cilia, which protrude 

 through the enveloping sac ; they vary in form, but are usually some- 

 what pear-shaped, and contain a starch-granule, a reddish-brown 

 pigment-spot, and one or two contractile vacuoles, the cilia being borne 

 at the narrow anterior hyaline end. The internal green globules are 

 young individuals formed within the parent, which thus constitutes a 

 colony or cainobe ; but all the cells which make up the colony are not 

 equivalent as respects their reproductive power. The larger number of 

 the cells are sterile or purely vegetative, while a much smaller number,, 

 developed at particular spots in the colony, are generative, these again 

 being differentiated into non-sexual propagative and male and female 

 reproductive cells. The sterile cells are the peripheral or swarm-cells, 

 2-3 mm. in diameter, which precisely resemble in structure Chlamydo- 

 coccus, the motile stage of Pleurococcus, or the zoospores of many 

 algae. The gelatinous membrane which envelopes each of these swarm- 

 cells is pierced by a number of canals, which lie nearly in one plane, 

 and which are filled by green or colourless extensions of the proto- 

 plasmic interior. Since the canals of adjoining swarm-cells correspond 

 in position, they appear as if they were all connected together by a 

 network of fine reticulations. The membrane is also perforated by two 

 pores, through which the vibratile cilia protrude into the surrounding 

 water. These bodies present the unusual pl^nomenon of cells endowed 

 with spontaneous power of motion, which have, nevertheless, as far as is 

 known, no reproductive function, and are therefore not properly called 

 zoospores. The non-sexual propagative cells, zoospores or partheno- 

 spores, are similar in structure to the sterile swarm-cells, but from two 

 to three times their diameter. Very early in the development of the 

 young colony the contents of the mother-cells of the zoospores begin 

 to divide by repeated bipartition, all in the same colony being usually 

 at the same stage of development at one time ; the daughter-colonies,, 



