272 



COLLEGE ZOOLOGY 



motion and is largely responsible for the dispersion of the species, 

 since with its help the larvae may travel long distances. The 

 primary germ-layers (ectoderm and entoderm) arise either by the 

 imagination of a blastula (Fig. 198, B) or by the growing over 

 of certain cells (epibole, Fig. 198, C). The mesoderm originates 

 in two primitive mesoderm cells derived from one of the larger 



FIG. 198. Stages in the development of mollusks' eggs. A, cleavage of 

 the egg of Crepidula, showing the origin of the first mesodermic cell (mes). 

 ma, macromeres; mi, micromeres. B, frontal section of an embryo of Paludina, 

 showing gastrulation by the invagination of a blastula (embolic). mes., meso- 

 derm bands; ud., archenteron; ., velum. C, an embryo of Crepidula, showing 

 epibolic gastrulation. bl. blastopore; ec, ectoderm; en, entoderm. (A and 

 C, from Lankester's Treatise, after Conklin ; B, from Korschelt and Heider, after 

 Tonniges.) 



cells (macromeres) of the cleavage stage (Fig. 198, A, mes). 

 Two mesoderm bands (Fig. 198, B, mes) are produced by the mul- 

 tiplication of the primitive mesoderm cells. 



The Position of the Mollusks in the Animal Kingdom. - - We 

 are not at all certain as to the relations of the Phylum MOLLUSCA 

 to other phyla. Some investigators have sought to derive the 

 mollusks from turbellarian-like ancestors. Considerable im- 

 portance is attached to the presence of a trochophore in the de- 



