INTRODUCTION TO THE METAZOA 



91 



spongelike characteristics: (1) flagellated 

 collar cells and (2) amoeboid wandering 

 cells. It is not difficult to imagine Protero- 

 spongid developing into a sponge. Sponges, 

 however, do not seem to occupy a place in 

 the main line of evolution. Another type of 

 metazoan ancestor could have been an 

 organism similar to a spherical protozoan 

 colony such as Volvox (Fig. 22). The blas- 

 tula stage (Fig. 41) is represented in the 

 development of many metazoans. But how 

 could the metazoans have evolved from a 

 hollow ball of cells? Invagination may have 

 occurred, resulting in a gastrula with two 

 layers of cells and a cavity, the gastrocoel 

 (Fig. 41). However, the arguments in favor 

 of this hypothesis are not impressive. Many 

 coelenterates (p. 106) resemble a modified 

 gastrula such as in the adult hydra, others 

 resemble the hollow ball filled with cells as 

 in the embryonic stage of hydra (Fig. 56, p. 

 116). The metazoans may have developed 

 from a type of larva, for example, the 

 planula (p. 119), which is characteristic of 



many of the lower metazoans. The origin of 

 the Metazoa from a two-layered primitive 

 planula is a hypothesis which many biolo- 

 gists favor (Fig. 430). The subject will be 

 discussed in some detail after more has been 

 learned about the various groups of Meta- 

 zoa. 



SELECTED COLLATERAL 

 READINGS 



Baitsell, G.A. Human Biology. McGraw-Hill, 



New York, 1950. 

 Huettner, A.F. Comparative Embryology of 



the Vertebrates. Macmillan, New York, 



1949. 

 Kimber, D.C., Gray, C.E., Stackpolc, C.E., 



and Leavell, L.C. Textbook of Anatomy and 



Physiology. Macmillan, New York, 1956. 

 Maximow, A.A., and Bloom, W. A Textbook 



of Histology. Saunders, Philadelphia, 1957. 

 Stiles, Karl A. Handbook of Histology. Blakis- 



ton Division, McGraw-Hill, New York, 



1956. 



