INVERTEBRATA. 433 



differentiation of ovum and spermatozoon, but is not so ; 

 it is a modification of the process peculiar to the Giliata. 



METAZOA. 



All other animals are united together in contrast to the 

 Protozoa under the name of Metazoa, and are distinguished 

 by the fact that the organism is a compound of cells 

 arranged in layers differing from one another in character 

 and functions. In all Metazoa at least three kinds of cells 

 are present : in the first place there are reproductive cells 

 (gametes) and somatic cells, the former alone capable of 

 reproducing the organism, usually after conjugation. The 

 somatic cells are differentiated at least into an internal 

 digestive and an external sensitive layer. The gametes are 

 always differentiated into ova and spermatozoa, mega- 

 gametes and microgametes, and the fertilised ovum or 

 zygote undergoes segmentation leading to the formation 

 of distinct layers of cells. The soma or body often exhibits 

 various kinds of asexual reproduction such as fission or 

 gemmation, but this process is due to the growth of por- 

 tions of the body tissues, not of single cells. Prof. Sollas 

 takes the view that the Sponges or Porifera should be 

 regarded as quite separate from the rest of the Metazoa, 

 and that they were evolved independently from a special 

 group of the Flagellata : he therefore proposes to give them 

 the name Parazoa, distinct from both Protozoa and Metazoa. 

 As generally understood, however, the term Metazoa 

 includes Sponges. 



PHYLUM COELENTERATA. 



The essential peculiarity of this Phylum is that the body 

 is saccular, containing only one cavity, which, however 

 much it may be complicated or differentiated into canals 

 and chambers, is continuous throughout, and has only one 

 principal aperture, the mouth. Other small apertures may 

 exist in some cases, but there is never a large number of 

 small pores in the walls of the body through which a 

 current of water passes as in sponges. The body is usually 

 ZOOL. 28 



