THE MASTIGOPHORA 267 



occurs in the life-cycle of Copromonas. In Mastigina, on the other 

 hand, the monad form developed from the zygote apparently does 

 not multiply by fission, but develops directly into the adult form 

 perhaps a more primitive state of affairs. 



A very instructive series is furnished by the colony-forming 

 Phytomonads of the family Volvocidce. At one end of the series 

 are primitive types, such as Stephanosphcera, where the colony is 

 composed of eight monad individuals, all alike, which may be 

 agamonts in one colony or gamonts in another. Each agamont 

 multiplies by fission to form eight small cells, which remain con- 

 nected together and grow into full-sized monads, thus giving rise 

 directly to new colonies. In the gamont-colonies each gamont 

 (gametocyte) gives rise by multiple fission to a large number of 

 minute biflagellate swarm-spores, the gametes, which are set free and 

 copulate. The syngamy is perfectly isogamous. The zygote grows in 

 size, and finally multiplies to form the eight monads of a new colony. 



At the other end of the series are the species of the genus Volvox, 

 in which the colony is composed of a great number of individuals, 

 which may be of three kinds, not necessarily all present in the same 

 colony : (1) The ordinary " somatic " monads, locomotor and 

 trophic in function, which do not reproduce themselves in any way ; 

 (2) agamonts, so-called " parthenogonidia," which multiply by 

 fission to form daughter-colonies ; (3) gamonts or gametocytes, 

 which are sexually differentiated as " microgonidia " and " macro- 

 gonidia." The microgonidia produce by multiple fission a swarm of 

 small biflagellate microgametes, comparable to the gametes of 

 Stephanosphcera. In the macrogonidia, on the other hand, multi- 

 plicative processes are in abeyance, and each becomes a single, ovum- 

 like macrogamete, which is fertilized by the relatively minute 

 microgamete. Thus, the syngamy in Volvox is anisogamous to 

 the highest degree ; and, as in other cases among Protozoa, this 

 condition appears to have arisen from a primitive isogamy in which, 

 in both sexes, the gametocytes sporulated to produce a swarm of 

 minute gametes, by the process of sporulation becoming altogether 

 suppressed in one sex namely, the female while retained in its 

 primitive form in the other. The colonies of Volvox, with their 

 differentiation of individuals, exhibit a condition transitional to 

 that of the Metazoa. The trophic, non-reproductive individuals, 

 taken as a whole, may be compared to the Metazoan soma, the repro- 

 ductive individuals to the germen. In Pleodorina calif ornica dis- 

 tinct male, female, or parthenogenetic colonies occur (Chatton), as 

 is the case in some species of Volvox. 



Classification. The Flagellata are classified in different ways by different 

 authors, and in the present state of our knowledge of the group no system 

 can be regarded as in any way final. As in other groups of Protozoa, there 



