44 MANUAL OF ZOOLOGY SECT. 



some forms two individuals come together and become 

 completely fused, the process being known as conjugation?- 

 the body formed by the union of the cells being known 

 as a zygote. The protoplasm of the latter divides by 

 multiple fission into very minute spores. These, when first 

 liberated by the rupture of the zygote, are mere granules, 

 but soon the flagella are developed. In some cases the 

 conjugating cells or gametes are of two sizes, union always 

 taking place between a large cell or megagamete and a small 

 cell or microgamete. In Volvox, which is a free-swimming 

 spherical colony, this difference between the two sets of 

 conjugating cells reaches its extreme (Fig. 17, , P\ G), pro- 

 ducing a condition of things closely resembling what we 

 find in tire sexual reproduction of higher forms. Certain 

 of the zooids enlarge and form megagametes, others divide 

 repeatedly and give rise to groups of microgametes, each 

 of the latter having the form of a rod-like body with two 

 flagella. The microgametes escaping, swim about freely and 

 conjugate with the motionless megagametes to form a zygote, 

 which, after a time divides to give rise to a new colony. 



Mastigophora occur under the most various conditions, 

 to some of which reference has been already made. Many 

 kinds live in fresh water ; others are abundant in the sea. 

 Noctiluca and others among the marine forms are phos- 

 phorescent, and are usually the agents by means of which 

 the diffuse phosphorescence of the sea is produced. Others, 

 again, are saprophytes, while others are parasites of higher 

 animals. 



2. THE INFUSORIA 



Often to be found in great numbers, in stagnant pools, 

 organic infusions, &c., is Paramoecium, the " slipper-shaped 

 1 Conjugation has also been observed to occur in many Rhizopods. 



