258 THE PROTOZOA 



of this kind leads to the formation of a colony, which may attain 

 to dimensions relatively large, though composed of individuals 

 of minute size. The colony may be free-swimming or fixed, and 

 in the latter case is frequently arborescent in form. In many cases 

 the colonies of Flagellata show a differentiation of the constituent 

 individuals into vegetative and generative individuals the former 

 not capable of reproduction, but purely trophic in function ; the 

 latter destined to be set free, and to produce new colonies, with or 

 without going through a process of syngamy. 



Bionomics. In their modes of life the Flagellata exhibit all the 

 four types described in Chapter II. (p. 13), different forms being 

 holozoic, holophytic, saprophytic, or parasitic ; and one and the 

 same form may live in different ways during different periods of its 

 life-history, according to circumstances. 



The parasitic flagellates have attracted a great deal of attention of recent 

 years, on account of their importance in causing disease in man and animals. 

 Ectozoic parasites may occur in aquatic forms, as for example Costia, para- 

 sitic on the skin of fishes. The entozoic forms are parasitic for the most 

 part in the digestive tract, or in the blood and lymph of their hosts. Parasitic 

 flagellates are found in the intestines of practically all classes of the Metazoa, 

 and especially in arthropods and vertebrates ; those parasitic in blood and 

 lymph are found especially in vertebrates, and constitute an important 

 group commonly termed as a whole the Hsemoflagellates, to which a special 

 chapter will be devoted. From forms which were probably parasitic originally 

 in the blood have arisen secondarily forms parasitic in cells which in their 

 intracellular phase lose their flagellum entirely (Leishmania). 



Many of the intestinal flagellates, especially in vertebrates, are probably 

 not true parasites at all, but for the most part scavengers. In any case their 

 pathogenic role appears to be very limited ; but in some cases a pathological 

 condition of the host may be combined in a suspicious manner with great 

 numbers of the parasites (compare Bohne and Prowazek, Noc). It is worthy 

 of note that in some cases an intestinal parasite may pass from the intestine 

 into the blood or lymph under pathological conditions of the host. This 

 condition seems to have been noticed first by Danilewsky, who described 

 cases of frogs and tortoises which had been kept long in captivity and were 

 in bad condition, thin, and with cedematous swellings in the muscles and 

 transudation of lymph into the peritoneal cavity ; in such animals there 

 were found in the blood and lymph, especially in the oedemata and trans- 

 udations, abundant flagellates of the genus Hexamitus ( =0ctomitus, Fig. 116), 

 of a species which in normal, healthy animals is found only in the intestine. 

 A number of similar cases have been recorded by Plimmer (383, and Presi- 

 dential Address to the Royal Microscopical Society, 1912), who found both 

 Octomitus and Trichomonas in the blood of various batrachia and reptiles. 

 The conditions under which these intestinal parasites pass into the blood 

 appears to be strictly comparable to those under which the Leydenia-iorm 

 of Chlamydophrys passes into the ascitic fluid (p. 237). Whether in such 

 cases the migration of the parasite is the cause of the diseased state of the 

 host, or whether, as seems more likely, the abnormal condition of the host 

 gives the parasite an opportunity of spreading into fresh pastures, must 

 remain for the present an open question ; but, according to Plimmer, the 

 presence of intestinal flagellates in the blood-circulation is associated with 

 definite and recognizable lesions of the intestinal wall. In any case, the 

 fact that intestinal flagellates can pass into the blood is a point which is 

 probably of phylogenetic as well as of practical importance (p. 322). 



