GENERAL ZOOLOGY 



Volvox itself stands at the apex of this series. A colony of V. globator, typical 

 of the genus, consists of some hundreds or thousands of cells, arranged in a 

 single layer about the periphery of a hollow ball (Fig. 9.3). Each of the 

 individuals composing the colony lies in a polygonal segment of the common 

 matrix. In favorable preparations,' each can be seen to be interconnected 

 with its neighbors by delicate protoplasmic bridges. These interconnections 

 presumably mediate coordinating influences which enable the relatively huge 

 colony to progress through the water in a directed fashion. They also make 

 possible the maintenance of a measure of integration of the activities of the 

 component individuals. Reproduction in Volvox is a function of only a rela- 

 tively few individuals, which either enlarge and sink into the central cavity 

 to produce daughter colonies by repeated asexual divisions or similarly en- 

 large and transform into either eggs or sperm bundles. Syngamy between 

 these anisogametes results in the formation of a zygote which secretes about 

 itself a heavy cyst wall and is thus enabled to survive conditions which result 

 in the death of the parent colony. The significant feature of Volvox is the fact 

 that, as in the Metazoa, only a relatively few cells retain the power of repro- 

 duction and can be termed germ cells. The vast majority of the component 

 cells are concerned not with the maintenance of the species through reproduc- 

 tion but only with the maintenance of the individual (or colony) through 

 their metabolic activities. Such a loss of physiological balance on the part 

 of the somatic cells is characteristic of the Metazoa; it is thus possible 

 to regard Volvox, and in the same sense Pleodorina, as multicellular organisms 

 rather than as colonies of protozoan cells. 



From our accounts of the Protozoa in the preceding chapter, it is clear that 

 there are no gross discontinuities between the vital functions of Protozoa and 

 those of Metazoa, aside from the fact that the protozoans carry on all their 

 activities within the confines of a single cell. Consideration of the series of 

 colonial protozoans just described emphasizes the fact that the development of 

 colonial organization in at least one group of modern Protozoa has progressed 

 to the point at which, in effect, multicellularity has been attained. Thus it 

 appears that the capacity of developing multicellularity is not lacking in 

 protozoans. We may conclude that through some similar persistence of 

 aggregations of one-celled animals, with the gradual emergence of the char- 

 acteristics of an integrated organism from the collective activities of its com- 

 ponent, originally independent units, a point was reached making possible 

 the rise of more complex many-celled animals. As previously indicated, the 

 next step involved in this progression, after the differentiation of somatic and 

 germinal cells, is the further specialization of somatic cells into specific struc- 

 tural and functional types, each capable of performing with added efficiency 

 one or a few special functions. 



The two phyla now to be considered, the Mesozoa and the Porifera, repre- 

 sent the simplest metazoan types known to exist. In these it will be seen that 

 the specialization of somatic cell types, and the division of labor among them, 

 has progressed to a considerable degree. Unfortunately, in the absence of 



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