78 COELENTERATA [CH. 



of lateral fringes on the walls of radial pouches and the union of 

 these fringes with one another. 



When the eggs fall out of the mouth they are caught in little 

 pockets and there develop into little planulae. These, as usual, 

 become free and swim about, and finally each fixes itself and 

 develops into a little polyp, called a Hydra-tuba, not unlike a 

 Hydra in appearance (A, Fig. 34), but there are nevertheless 

 important points of difference. Thus there is no oral cone but a 

 flat oral disc, in the centre of which the mouth opens into the 

 coelenteron. The latter has four ridges projecting into it, the lower 

 edges of which are free while the upper ones are joined to the oral 

 disc,- These ridges being produced by the folding of the endoderm 

 layer they are double and contain between their two limbs a space 

 filled with jelly. Into this space a prolongation of the ectoderm of 

 the mouth disc grows down so as to form a " septal funnel." The 

 cells composing the septal funnel secrete longitudinal muscular 

 fibrils, and thus four powerful septal muscles are formed which 

 serve to shorten the Hydra-tuba. The hydroid persons of the 

 Hydromedusae have also longitudinal muscles but these are dis- 

 posed in a uniform sheet round the polyps in question and belong 

 to the ectoderm cells forming the sides. During a large part of the 

 year the Hydra-tuba multiplies by budding, just as a Hydra does, 

 but at certain seasons it undergoes a very remarkable change (B and 

 C, Fig. 34). The oral disc flattens out very much and its edges 

 become drawn out into lobes, the tentacles at the same time drop- 

 ping off. A short oral cone is developed from the centre of the disc, 

 the mesenteries become perforated and finally the whole flattened-out 

 top of the Hydra-tuba breaks off and swims away. This is known 

 as an Ephyra larva (D and E, Fig. 34). It leads a free life and 

 gradually develops into a large jelly-fish. But long before the 

 primary oral disc has become free, the part of the Hydra-tuba next 

 below has been growing out so as to produce a similar disc. This 

 process, called Stabilisation (Gr. o-rpo/SiXo?, a whorl), is repeated 

 until the Hydra-tuba resembles a pile of saucers, in which state it 

 is called aScyphistoma (Gr. o-*v<os, a saucer). 



We can get some idea as to how this extraordinary development 

 may have arisen on the following hypothesis : The original Scy- 

 phozoan was probably an organism like a hydroid of the Hydrozoan 

 genus Tubular ia with a wide head and narrow base. In this top the 

 generative organs were developed, and when the eggs became ripe 

 it broke off and wandered away in order to disperse the species. 



