Ill] 



HYDROMEDUSAE 



65 



feeds on the small organisms of all kinds, both plants and animals, 

 which are found at the surface of the sea. After some time it com- 

 mences to give rise either to eggs or to spermatozoa, which usually 

 develop in exactly the same way in which they developed in Hydra> 

 i.e., from the interstitial cells of the ectoderm. The accumulations 

 of these cells, called gonads or generative organs, are borne 

 either on the under side of the bell (3, Fig. 25), or on the sides of 

 the manubrium, and it is a curious fact that those Medusae which 

 have them in the former position usually possess ear-sacs, whereas 

 when the gonad is situated on the oral cone, ear-sacs are never 

 present, but eyes may be. The eggs and spermatozoa are both shed 

 out into the water and coalesce there, and the fertilised egg develops 



A C 



FIG. 28. The ciliated larva or Planula of a Hydrornedusan, Clava Squamata. 



Magnified. From Allman. 

 A & B. Swimming about in the sea. C. Coming to rest on a rock. 



D. Developing tentacles, oral cone and stolon. 1. Tentacles. 2. Oral 



cone. 3. Stolon. 



into a little oval larva, termed a Planula (Fig. 28), without 

 tentacles or mouth, and covered all over with cilia. It consists at 

 first of a hollow vesicle of ectoderm cells, which later becomes filled 

 with a solid plug of endoderm. This little creature swims about for 

 a while and then attaches itself by one end to a stone or a piece of 

 sea-weed. The attached end flattens out (C and D, Fig. 28), but 

 the rest of the animal lengthens and a mouth and tentacles appear 

 at the free end and the endoderm becomes hollowed out, so that 

 the creature takes the form of an unmistakable hydra-like organism. 

 It then begins to bud out a branch called a stolon which creeps along 

 the substratum. From this other polyps will arise, each of which 

 has only to bud in order to reproduce the colonial stock from which 



S. &M. 5 



