CLASSIFICATION OF THE ANIMAL KINGDOM. 207 



which may or may not be distinct from the first aperture of the 

 Gastrula (monostomatous). 



Metazoa polystomata. — The former division comprises only the 

 Sponges (Porifera or Spongida), in which, as the remarkable re- 

 searches of Haeckel (" Monographic der Kalk-Schwamme ") have 

 shown, the walls of the deeply cup-sliaped Gastrida become per- 

 forated by the numerous inhalant ostioles, while the primitive 

 opening serves as the exhalant aperture. 



The latter division includes all the remaining forms, which may 

 be grouped together as Metazoa monostomata. Among these, two 

 primary groups are distinguishable, of which the second exhibits 

 an advance in organization upon the first. In the first, the pri- 

 mitive aperture of the Gastrula becomes the permanent moutli 

 (Archaeostomata). In the second, the permanent mouth is a 

 secondary perforation of the body-wall (Deuterostomata). 



1. The ArchoBostomata. — It is now well established that the 

 aperture of the Gastrula becomes the oral aperture of the adult in 

 the Coelenterata, which group includes animals differing much in 

 grade of organization, from the simple Hydra to the complex 

 Ctenopliore, but all manifestly exhibiting variations of one funda- 

 mental type. 



In most of the Ilydrozoa, the ovum passes into a solid Morula, 

 which, as in the Porifera, becomes difl:erentiated into an epiblast 

 and a hypoblast. The central cavity of the latter opens at one end, 

 and thus far the Gastrida of the Ilydrozoa is very like that of the 

 sponges ; but the aperture produced in this manner becomes the 

 mouth ; and if, as not unfrequently happens, apertures are formed 

 elsewhere, they do not serve the purpose of taking in food. In 

 such Ilydrozoa as have thickened body-walls, hollow prolongations 

 of the hypoblast extend into the blastocoele, and are surrounded 

 by a mesoblastic tissue. These prolongations may become branched 

 and anastomose, resembling vascular canals ; but they remain 

 permanently in connexion with the alimentary cavity. The re- 

 productive elements are developed in the body -wall, and usually 

 in cascal outwardly projecting processes of that wall, which dehisce 

 and set free the ova and spermatozoa upon the outer surface of 

 the body. 



The Actinozoa, while presenting the same continuity of the 

 cavity of the body with the alimentary cavity which is exhibited 

 by the Hydrozoa, differ from them in two respects. The com- 

 mencement of the alimentary canal is, as it were, sunk in the 



