178 THE INVERTEBRATA 



The eggs are fertilized as soon as they become free by spermatozoa 

 from another individual which are drawn^into the mouth along with 

 the food. They pass through the canals to the opening on the oral 

 arms (Fig. 132, o.o.a.) and undergo the first stage of their development 

 enclosed in pouches at the side of the oral grooves. Little opaque 

 patches along the side of the lips are to be seen with a lens, and when 

 dissected out they prove to be masses of planula larvae. The planula^ 

 is eventually set free, but soon attaches itself to stone or weed and 

 develops into a small polyp, without perisarc, the hydratuba, which 



Fig. 134. Strobilation of Aurelia aurita. From Sars. A, Hydratuba on 

 stolon which is creeping on a Laminaria. The stolon is forming new buds at 

 I and 2. B, Later stage or scyphistoma, x 4. The strobilation has begun. 

 C, Strobilation further advanced, x 6. D, Free-swimming ephyra stage, 

 showing first appearance of unbranched adradial canals, x 7-5, seen from 

 below. E, The same seen in profile, X7'5. 



eventually grows sixteen long and slender tentacles. Internally this 

 stage has the same structure as Lucernaria with four interradial 

 mesenteries, which are invaded by vertical ectodermal pits, and form 

 perradial pouches between. At the base of the hydratuba a horizontal 

 stolon grows out, and off this fresh hydratubae may be budded 

 (Fig. 1 34 A). They may separate from the parent as in Hydra. During 

 the winter the whole hydratuba is segmented by transverse horizontal 

 furrows. This process is termed strobilation (Fig. 134 B). In each 

 of the disc-like segments so produced, marginal growth at once begins, 



^ In Aurelia the formation of the planula sometimes takes place by in- 

 vagination of the blastula. 



