394 



ZOOLOGY. 



umbrella to the other. This membrane is at last broken 

 through and the remnant of it forms a circular shelf pro- 

 jecting inwards from the margin of the umbrella. This 

 shelf is called the velum, and is permanent and conspi- 

 cuous in most hydroid medusae, but in Obelia it diminishes 



BELL 



RUDIMENT 



Fig. 201. DIAGRAM OF THE DEVELOPMENT OF A MEDUSA FROM THE BLASTOSTYLB. 



The successive stages are numbered 1 to 6 and are represented as all connected 

 with the same blastostyle, 



and becomes quite vestigial. It is therefore not repre- 

 sented in our diagrams. When the medusa bud has 

 acquired its final shape the mouth breaks through at the 

 extremity of the manubrium and finally the stalk is broken, 

 the aperture into the enteric cavity formed by the rupture 

 closes up, and the little medusa becomes free and escapes 

 from the gonotheca through the aperture at its end. 



8. Comparison of Hydroid and Medusa. By comparing 

 the structure of the two it can be seen that the medusa is 



