176 



ZOOLOGY. 



separate individuals. After fertilization cleavage is total but 

 sometimes not equal. A blastula is formed which is often 

 converted into a peculiar, free swimming, ciliated larva 

 (planula), consisting of a two-layered sac with no opening. 

 This condition may arise by the closing up of an ordinary two- 

 layered gastrula (as in Aurelia). In other cases the entoderm 



o 



FIG. 82. Diagrams illustrating development in some of the hydroid types. A, 

 blastula in which the entoderm (ent.) is produced by proliferation from ectoderm (ect.), 

 B, ciliated planula formed by the continuance of this process. A split in the entoderm 

 furnishes the beginning of the gastrovascular cavity (g) of the adult. C, more 

 mature condition, in which the planula has become fixed: /, foot or attached end; 

 o, oral or free end at which the tentacles and mouth will be developed. 



Questions on the figures. How does this blastula differ from the 

 typical blastula in the formation of entoderm? What is a planula? Is a 

 gastrula formed? After an opening forms at the oral end what likeness 

 is there in the adult to a gastrula? What changes would C need to 

 undergo to become essentially similar to Hydra? 



may be formed by cells budding into and finally lining or even 

 filling the segmentation cavity of an ordinary blastula (Fig. 

 83), resulting in a quite similar condition. The planula after 

 a brief free life becomes attached by one pole and becomes 

 elongated; a mouth surrounded by tentacles is formed at 

 the other. . Thus it assumes the typical polyp form. In nearly 

 all species the polyps may produce new individuals by buds 

 either from the wall of the polyp or from special organs 

 (stolons, or runners}. If, when these are mature, they sepa- 

 rate from the parent no colony is formed. More commonly 



