FROM ONE CELL TO MANY CELLS 



69 



In some organisms two of these types of colonies may be combined. 

 The cells may be in globular masses (spheroid type), though not iml^edded 

 in jelly, and several of these masses joined by a branching stalk (Fig. 53). 



Choice of Colony to Illustrate Metazoan Origin. — If it is assumed, in 

 tracing probable lines of descent, that the colonial theory is correct, 

 which of these colonial types is most likely to represent the early evolution 

 of the metazoa? The massive compact form of most of the metazoa 

 suggests that the linear and dendritic colonies may be left out of consider- 

 ation. Of the other two types, each has something in its favor. The 

 fact most favoring the gregaloid colony is that in one of 

 the best known organisms of that kind, Proterospongia 

 (Fig. 50), each cell at the surface bears a delicate proto- 

 plasmic collar around its one flagellum. Such a collar, 

 surrounding a flagellum, is found on certain internal 

 cells of the sponges (Fig. 33, page 52), which constitute 

 one of the simplest groups of metazoa. Some biologists 

 have inferred from these collared cells that the earliest 

 metazoa may have been in some degree spongelike and 

 that they came from colonies somewhat like present-day 

 gregaloid colonies. 



The spheroid type of colony is favored by its greater 

 abundance at the present time. Most of the spheroid 

 colonies consist of cells bearing flagella, and many 

 students of protozoa have held that the flagellate forms 

 are the most primitive of the single-celled animals, 

 which is another pair of facts in favor of the spheroid 

 colony. Furthermore, the spheroid colonies lead directly 

 to other forms that may, as we shall see, be used to illustrate later steps 

 in the evolution process. 



This reasoning may not be correct, biit many biologists in the past 

 have followed it and concluded that the metazoa probably arose from 

 a single-celled organism, bearing some resemblance to modern flagellates 

 (Fig. 54), through the formation of colonies. 



The First Differentiation. — In all the colonies described, the cells of 

 one group are all alike, at least potentially. In Proterospongia (Fig. 50) 

 they may seem to be of two kinds, since the cells in the interior of the 

 jelly mass do not have collars. This is not a real difference, however, 

 for the cells take turns coming to the surface, where they feed, and while 

 at the surface develop a collar and flagellum, which they lose when they 

 retreat to the interior. 



Now, the chief distinguishing mark of the metazoa is that their cells 

 are not all alike. In the evolution of the multicellular organisms there 

 must have been a differentiation of the adhering cells into two or more 



Fig. 54 .— 

 Chlamydomonas, 

 illustrating a 

 primitive type of 

 organism from 

 which colonies 

 and later met- 

 azoa may have 

 arisen. 



