THE MAIN LINES OF ANIMAL EVOLUTION 



theoretical importance lies in the fact that sponges have similar cells, 

 called choanocytes, and that colonial choanoflagellates, such as Protero- 

 spongia, resemble simple sponges. 



The most highly specialized flagellates are probably the orders Poly- 

 mastigina and Hypermastigina. The polymastigotes are generally parasitic 

 on the digestive tract of arthropods or vertebrates. The simpler members 

 generally have four flagella, but the more complex members may have 

 large numbers. There may be only a single nucleus, or there may be 

 many. Complicated cytoplasmic organelles may be present. Reproduction 

 is generally reported to be by simple fission, but Cleveland has reported 

 sexual reproduction in some species, including the unique feature of re- 

 duction of the chromosomes at a single division. The hypermastigotes are 

 morphologically the most complex flagellates. There are numerous flagella 

 arranged in definite ways. Cytoplasmic organelles are very complex ( Fig- 

 ure 45 ) . All hypermastigotes are parasites or commensals in the digestive 

 tract of termites or cockroaches. They are essential to the nutrition of the 

 hosts, for which they digest cellulose, for it has been shown that the host 

 starves if the protozoans are removed experimentally. 



The order Rhizomastigina is of especial interest because the members 

 of the order appear to be intermediate between the Flagellata and the 

 Sarcodina, ameba and its allies. Many flagellates are capable of ameboid 

 movement, but the members of this order, although they do possess a 

 flagellum, are also permanently ameboid, and so they make a nice con- 

 necting link between the Flagellata and the Sarcodina. Mastigamoeha is 

 a good example. 



Ameba and Its Allies. The class Sarcodina is also highly diversified, 

 and it is by no means certain that it is really a unified group. That is, 

 different orders of the Sarcodina may have arisen from different flagellate 

 ancestors. By far the best known member of this class, or perhaps of any 

 protozoan class, is Amoeba proteus, the familiar study material of every 

 elementary biology laboratory, and the traditional example of a primitive 

 animal. A. proteus is quite typical of its order, the Lobosa. But, in addition 

 to such simple, free-living, ameboid organisms, the Lobosa includes para- 

 sites like Entamoeba histolytica, which parasitizes the digestive tract of 

 man; and there are free-living but shelled organisms such as Difflugia. 

 The nutrition of all of these, including the parasites, is holozoic. Repro- 

 duction by binary fission is the rule, but sexual reproduction has been 

 reported for one species of Amoeba, and it is common among the shelled 

 species. The order Lobosa is generally regarded as a terminal group in 

 evolution, yet the idea that the Myxomycophyta or the Eumycophyta or 

 both may have been derived from this group is gaining prestige among 

 mycologists (see Chapter 8). And some zoologists treat the Myxomyco- 

 phyta as protozoans under the name of Mycetozoa. 



The remaining orders of the Sarcodina are all terminal groups which 

 are characterized by elaborate and often beautiful calcareous or siliceous 

 shells, and by many slender, semipermanent pseudopodia, the character- 

 istics of which are peculiar to each order. These are the Foraminifera and 

 the Radiolaria, both of which are abundantly represented in the fossil 



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