80 



FIRST LESSONS IN ZOOLOGY. 



The medusae arise from little bud-like swellings on the 

 young or Hydroid (Fig. 24, ); these enlarge, and finally 

 become detached and swim about as at Fig. 25. 



The growth of a jelly-fish from the Hydra-stock is a very 

 striking fact. It will be seen that the offspring of the 

 Hydra is unlike its parent, but exactly like its grand- 

 parent. There is a generation of Hydras, and a generation 

 of jelly-fish, and these alternate, the Hydra not producing 

 young Hydras, but young jelly-fishes. 



Our common large jelly-fish or ' ' sun-fish " so often thrown 

 ashore on sandy beaches is the Aurelin (Fig. 29). It grows 



FIG. 26. Larva or 

 Scyphistoma of 

 Aurelia. (Much 

 magnified.) 



FIG. 27. Strobila of 

 Aurelia. (Magnified.) 



FIG. 28. Ephyra or ear- 

 liest free condition of 

 Aurelia. (Magnified.) 



eight or ten inches in diameter. Its tough, jelly-like, convex 

 disk is smooth above, but hollowed out beneath into a broad 

 stomach with a square mouth, the edge of which is minutely 

 fringed, and bears four fringed broad, short tentacles. On 

 the fringed margin are eight covered eyes situated in inden- 

 tations, which divide the disk into eight slightly marked 

 lobes. The four main water- vascular canals subdivide, as 

 seen in Fig. 29, into numerous branches, which connect 

 with the marginal vessel. 



The Aurelia spawns late in the summer. The eggs pass 

 out of the mouth into the water along the channelled arms, 

 and in October the ciliated sac becomes pear-shaped and 



