846 THE ANIMAL KINGDOM. 



and Volvox. One individual, the macrogonidium (female), is large and 

 ovoid, and in Volvox devoid of cilia ; the other (male) is elongate, 

 biflagellate, of a yellowish tint, grouped in colonies which are produced 

 by fission from a single large cell, and contain 16, 33, 64 individuals in 

 Ettdorina, or even 128 in some species of Volvox, The two sorts of indi- 

 viduals are contained in different colonies in Eudorina, Volvox Carteri, 

 and sometimes in V. minor, or in V. globator in the same colony. V. minor 

 is said to develope first one, then the other sort. Every individual in a 

 given colony of Eudorina is modified ; in Volvox only a certain number 

 which resemble at first the parthenogonidia above mentioned (p. 845), but 

 are as a rule much more numerous. The male colony is set free as a 

 whole in Eudorina and V. minor. After conjugation a double cyst is 

 formed by the zygote, which now developes haematochrome, and is set free 

 by the death and resolution of the original colony. In Volvox ( V. minor] 

 it appears to rest during the winter ; in spring its contents undergo fission 

 and are set free as a young colony of about 500 cells. 



Hypnocysts are formed by all Flagellata when the conditions of life 

 become unfavourable. They have thick and often multiple walls. In the 

 Chlamydomonads the cyst is formed within the envelope, which is then lost, 

 and in Volvocina (? Volvox itself) every individual of a colony encysts at 

 the same time. Resting green forms turn red. In a few instances encyst- 

 ation of the nucleus, plus a small quantity only of protoplasm, has been 

 observed, the rest of the protoplasm perishing 1 . Fission of the contents 

 of the hypnocyst occurs in Stephanosphaera and Haematococcus, in the 

 latter sometimes giving rise to Pleurococcoid crusts. 



The great majority of known Flagellata inhabit fresh water ; a few 

 are marine, and a few parasitic in Vertebrata, Arthropoda, especially In- 

 secta and Myriopoda, in some Mollusca and Nematodes, inhabiting 

 the digestion tract, blood-vascular system, or the epidermis like Bodo 

 necator, so destructive to young trout. They sometimes swarm in great 

 numbers imparting a colour to water, green, e.g. Euglena viridis, Haemato- 

 coccus, red, e.g. Euglena sanguinea, or even to snow, like the red Haemato- 

 coccus, or the yellowish-green Chlamydomonas Jlavovirens. The contents of 

 the reproductive cysts of some saprophytic species are capable of with- 

 standing a great heat without being killed, e. g. an Oikomonas 26iF. in a 

 moist condition, 3OOF. in a dry. An artificial adaptation of saprophytic 

 species to life at higher and higher temperatures has been successfully 

 carried out. The green Flagellata evolve oxygen under the influence of 

 sun-light, and how far they can endure darkness and live appears a 

 moot point. They are sensitive to light, being attracted or repelled by 



1 See Fisch, Z. W. Z. xlii. 1885, p. 72 (Chromulina), p. no (Arhabdomonas), p. 114 (Monas 

 guttula}. 



