Ill] CTENOPHORA 81 



within a pouch (7, Fig. 35). This branched tentacle is covered with 

 adhesive cells, there is one on each side of the animal. These last 

 mentioned canals are termed perradial. Each perradial canal gives 

 off two lateral branches termed interradial canals and each inter- 

 radial canal finally gives off two so-called adradial canals (6, Fig. 35) 

 which lead into the meridional canals running under the ribs, from 

 the cells lining which both ova and spermatozoa are produced, 

 Ctenophora being hermaphrodite. Between ectoderm and gutwall is 

 a gelatinous substance which differs from the jelly of other Coelen- 

 terata in having numerous cells with long processes embedded in it. 

 These processes connect the cells with one another and with the 

 ectodermal and endodermal cells. Many of them are contractile i.e. 

 have become muscular and thus the jelly of the Ctenophore is 

 traversed by a network of thread-like muscles. The commonest 

 British form is Hormiphora plumosa, which sometimes appears in 

 shoals in the seas washing the Atlantic coast of Britain on the 

 one hand and America on the other. The Ctenophora are good 

 examples of what are called pelagic organisms, that is to say, 

 organisms which pass their whole life from the egg to the adult 

 condition floating at or near the surface of the sea. Such organisms 

 are the only ones which are found in mid-ocean. Nearer the shore 

 the waters are filled by a profusion of other animals, but these turn 

 out on examination to be largely composed of forms which in some 

 period of their existence are adherent to or creeping on the bottom. 

 Other purely pelagic groups are the Siphonophora, the Trachy- 

 medusae and the Narcomedusae. 



The Ctenophora contain many forms which differ widely in 

 appearance from Hormiphora for instance the Cesium veneris y or 

 Venus's girdle, a beautiful transparent ribbon-like creature, a foot 

 or so in length and two or three inches wide. On close examination 

 the reason of this diversity of shape is found to be that the Cteno- 

 phora are not really radially symmetrical, but doubly bilaterally 

 symmetrical. That is to say, not only right and left sides are like 

 one another but also the back and belly are alike, but at the same 

 time different from the sides. The difference between back and 

 belly on the one hand and the two sides on the other is slight in 

 Hormiphora but very strongly marked in Cesium veneris. 



8. & M. 



I 



