CHAPTER TEN 



The Main Lines 

 of Anifnal Evolution 



As POINTED OUT ill Chapter 8, the Protozoa, the most primitive phykim 

 of animals, appears to have been derived from primitive flagellate algae, 

 and within single genera among the euglenoids there may be some species 

 which show predominantly plant characters and others which show pre- 

 dominantly animal characters. Thus there is a high probability that the 

 flagellates are close to the point of separation of the two kingdoms, if, 

 indeed, the separation is complete here, for there are flagellates like Trif- 

 panosoma which are undoubtedly animals; there are others like Chlamij- 

 domonas which are undoubtedly plants; and there is the great intermedi- 

 ate group, typified by Eiiglena, which defies any indisputable assignment 

 to the kingdoms. 



DIVERSIFICATION OF THE PROTOZOA 



Once the Protozoa were established, they became greatly diversified, and 

 their relationships are by no means certain. The class Flagellata itself 

 includes a wide range of structural and ecological types even when only 

 indisputably animal types are considered. Feeding may be by engulfing 

 food with pseudopodia, or by a simple mouth. The parasitic forms are 

 generally saprozoic. Several orders of flagellates deserve especial mention. 

 The order Protomonadina includes a wide variety of small, colorless 

 flagellates, tvpically with two llagella, one of which trails along the side 

 of the animal. Reproduction is always asexual. In the free-living members 

 of the order, this is by simple fission, but among the parasitic species there 

 may be multiple divisions which are diflicult to distinguish from spore 

 formation of the Sporo/.oa. It seems highly probable that the Sporozoa, 

 all of which are parasitic, were evolved from this group. But sexual re- 

 production was also developed among the ancestors of the Sporozoa. The 

 best known protomonads are the members of the genus Trypanosoma, 

 all of which are blood parasites of vertebrates. Quite as important for 

 evolution, however, are the choanoflagellates. These are protomonads 

 which have a protoplasmic collar encircling the base of the flagellum. Its 



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