INVERTEBRATA. 



385 



to two distinct nuclear materials combined in one nucleus 

 a somato-plasm which has only the potentiality of producing 

 (in this case) ectoderm-cells, and a germ-plasm which has 

 the potentiality of producing a complete Hydra. 



These two nuclear materials would correspond very closely 

 with the two which form the two very distinct nuclei 

 of Paramecium somato-plasm answering to meganuclear 

 material, germ-plasm to micronuclear. Attempts have 

 been made to distinguish between these two materials in 

 the nuclei of Metazoon cells, but without much success. We 

 need therefore say no more here, but would only call the 

 reader's attention once more to the comparison of the 

 life-histories of higher Protozoa and Metazoa on p. 283. 



13. Hydra and the Gastrula, The student will pro- 

 bably have already been struck with the general resemblance, 

 in respect of the broadest facts of structure, between Hydra 

 and the gastrula of Amphioxus. In each there are two 

 layers of cells with a central enteric (or archenteric) cavity 

 the outer layer in each case giving rise to sensory, nervous, 

 and protective structures ; the inner, to digestive structures. 

 Thus it may be said that Hydra remains permanently at 

 the gastrula stage, so far as general structure goes. On the 

 theory of evolution, the remote ancestors of Amphioxus 

 were probably permanent gastrulae, and Hydra may be 

 regarded as a representative of the group of animals from 

 which the ancestors of Amphioxus sprang. But we must 

 be careful to avoid the easy mistake of assuming that 

 Hydra closely resembles those hypothetical ancestors. Hydra 

 is by no means a simple gastrula its tentacles, nemato- 

 cysts, lack of cilia, fresh-water habitat, and crustacean diet, 

 are all features very unlikely to have been found in the 

 primordial gastrulse. The following words are as true now 

 as when they were written forty years ago by Huxley, who 

 first pointed out the similarity of Hydrozoa to Vertebrate 

 embryos : " All that can be justly affirmed is that the 

 Hydrozoon travels for a certain distance along the same 

 great highway of development as the higher animal, before 

 it turns off to follow the road which leads to its special 

 destination." 



ZOOL. 25 



