CHAPTER 



13 



THE PHYLUM MOLLUSCA 



Schizocoela and Enterocoela 



The major groups of eucoelomate animals constitute two great divergent 

 stocks, distinguishable on the basis of several fundamental differences. These 

 differences involve such features as the type of cleavage, the time in develop- 

 ment when the cellular precursors of the various organs become differentiated, 

 and the relationship of the blastopore to the axial polarity of the embryo 

 and the future adult. In addition, there are basic differences in the mode of 

 formation of the mesoderm and of the coelom, and it is from this particular 

 characteristic that the terms Schizocoela and Enterocoelo are derived. In 

 the schizocoelous forms, comprising the major phyla Mollusca, Annelida, and 

 Arthropoda and several minor phyla, the mesoderm of the adult arises by 

 proliferation of cells called mesoblasts, set aside early in the cleavage 

 process. The coelom forms as a result of the development of cavities within 

 the solid masses of mesodermal cells so produced. In the enterocoelous forms, 

 on the other hand, represented by the major phyla Echinodermata, Hemi- 

 chordata, and Ghordata, the mesoderm appears primitively as hollow out- 

 growths from the wall of the embryonic gastrocoel, and the coelomic cavities 

 develop as enlargements of the spaces within these evaginations. 



It should be understood that from the point of divergence of these two 

 stocks the phylogenetic tree of animals consists of two major branches, in 

 each of which evolution has proceeded independently of the other (see Fig. 

 7.3, p. 219). The schizocoelous branch culminates in the great phylum 

 Arthropoda, and the enterocoelous branch has reached its apex in the phylum 



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