CHAPTER III 



THE CCELENTERATES OR HYDROIDS: JELLY-FISH. ANEMONES 

 AND CORALS 



WE have seen that the sponge in its simplest form is a cylinder, 

 closed at one end where it is attached to a base, open at its free 

 end, with its walls perforated by minute openings or pores, and 

 composed of three layers of cell-structure ectoderm, mesoderm, 

 and endoderm this last layer consisting of collared flagellate cells. 

 Imagine a transformation to take place in which the minute open- 

 ings in the wall of the cylinder have disappeared, so that the 

 internal cavity now only communicates with the exterior by a 

 single terminal opening ; that the inner layer of tissue or meso- 

 derm has become converted into a very thin structureless layer 

 containing no cells, while the endoderm has lost its collared cells ; 

 and that a circle of waving arms or tentacles, formed of the same 

 layers as the body-wall, have grown out round the terminal open- 

 ing which now serves as the sole aperture for the reception of 

 food as well as for the discharge of effete matters. Such a trans- 

 formation completed, the resulting organism would serve as an 

 example of the general structure of the group of animals we are 

 now going to consider, while the organism itself would be a typical 

 Polyp or Hydroid. 



The animals grouped together and forming this great division 

 of the animal kingdom are characterised by the possession of a 

 single internal cavity (or ccelom), this body-cavity, which is the 

 primitive gut or intestine (enter on), only opening to the exterior 

 by a single aperture. It is to signify the possession of this im- 

 portant anatomical feature, absent in all the organisms we have 

 so far examined, that the name Coelenterata 1 was given to 

 this division in which are to be found the Hydra of our ponds, 

 the Sertularian Polyps of the sea, the large Jelly-fish, the graceful 

 1 Greek koilos t a hollow ; enteron, bowel. 

 39 



