184 



ZOOLOGY 



.(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 

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

 filling the segmentation cavity of an ordinary blastula (Fig. 

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

 a tirief free life becomes attached by one pole and becomes 

 elongated ; a mouth surrounded by tentacles is formed at the 



o 



FIG. 84. Diagrams illustrating development in some of the hydroid types. A, blastula in 

 which the entoderm (<.) 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 

 gastro- vascular 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 gas- 

 trula? What changes would C need to undergo to become essentially similar to 

 Hydra? 



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 separate from the 

 parent no colony is formed. More commonly the daughters 

 remain in association with the parent. The medusoid in- 

 dividual, often of a very much simpler type than that de- 

 scribed above (223), may be produced in a similar way from 

 a bud. This sexual individual may degenerate until it is little 

 more than a case for the sexual cells. It usually breaks its 



