II AURELIA 105 



of two layers of cellSj an outer ectoderm and an inner endoderm. It 

 will be seen that the gastrula in its fundamental characteristics agrees 

 with Hydra : the differences are rather differences in detail — ^the cup-like 

 body being in Hydra deepened to form a tube, the mouth being narrowed 

 into a minute pore, the body-wall being prolonged to form the tentacles, 

 and the component cells being specialized for different functions. The 

 fundamental similarity which underlies these, superficial differences 

 justifies the statement that the gastrula phase in the development of an 

 animal is in fact simply a temporary hydroid phase. 



In the case of Aurelia the gastrula undergoes a series of modifications 

 which culminate in a condition very much like that of Hydra even in 

 detail. The gastrula assumes a tubular form and the mouth becomes 

 narrowed into a minute pore (Fig. 44, C). It makes its way out of the 

 shelter in which it has developed so far and swims away through the 

 water by means of powerful cilia which have developed from its ectoderm. 

 The free-swimming larva presently attaches itself by its closed end to 

 some solid body, very often the frond of the large Tangle or Oar-weed 

 (Laminaria), and gradually takes the form of a little greyish-white creature 

 which was supposed by the older naturalists to be simply a marine species' 

 of Hydra and given the name Hydra tuba (Fig. 44, D). Features which 

 distinguish it from the true Hydra are the larger number of tentacles 

 and the fact that the endoderm undergoes an increase in area by forming 

 four folds which project into the coelenteron as four longitudinal ridges. 



This hydroid stage in the development of Aurelia is known as the 

 scyphistoma stage. The scyphistomas may sometimes be observed 

 during the autumn in untold myriads, dotted about on the fronds of 

 Laminaria, in our quiet sea-lochs. 



The scyphistomas feed actively, grow, and multiply by budding 

 during the autumn months but during the early winter a change begins 

 to come over them. Ring-like constrictions encircle the body and gradu- 

 ally deepening divide it into a pile of saucer-shaped structures one over 

 the other (strobila stage — Fig. 44, E and F). The margins of these 

 saucer-shaped bodies grow out each into eight lobes, while the 'Scyphi- 

 stoma tentacles upon the uppermost one degenerate and disappear. 

 Finally the saucer-shaped pieces break off one by one and swim away 

 as little star-shaped medusae (ephyra stage — Fig. 44, G) each with eight 

 lobes radiating outwards. Careful examination of the ephyra shows it to 

 be, notwithstanding its star shape, a young Aurelia medusa, for in a little 

 recess at the end of each of the eight arms there may be recognized the 

 characteristic sensory tentacle just like that of the adult (Fig. 44^ H, s.t.). 

 The ephyrae grow rapidly, the spaces between the rays becoming gradually 



