CHAPTER III. 



THE PORIFERA AND THE CCELENTERATA. 



1. The Porifera or Spongida. — It has been seen that, 

 in the Protozoa, the germ undergoes no process of division 

 analogous to the " yelk division " of the higher animals, and 

 to the corresponding process by which the embryo cell of 

 every plant but the very lowest becomes converted into a 

 cellular embryo. Consequently, there is no blastoderm ; the 

 body of the adult Protozoon is not resolvable into morpho- 

 logical units, or cells, more or less modified ; and the aliment- 

 ary cavity, when it exists, has no special lining. Moreover, 

 the occurrence of sexual reproduction in most of the Proto- 

 zoa is doubtful, and there is, at present, no evidence of the 

 existence of male elements, in the form of filamentous sper- 

 matozoa, in any group but the Infusoria ; and even here the 

 real nature of these bodies is still a matter of doubt. 



In all the Metazoa, the germ has the form of a nucleated 

 cell. The first step in the process of development is the 

 production of a blastoderm by the subdivision of that cell, 

 and the cells of the blastoderm give rise to the histological 

 elements of the adult body. With the exception of certain 

 parasites, and the extremely modified males of a few species, 

 all these animals possess a permanent alimentary cavity, 

 lined by a special layer of cells. Sexual reproduction always 

 occurs ; and, very generally, though by no means invariably, 

 the male element has the form of filiform spermatozoa. 



The lowest term in the series of the Metazoa is un- 

 doubtedly represented by the Porifera or Sponges, which, 

 after oscillating between the vegetable and the animal kimr- 

 doms, have, in recent times, been recognized as animals by 

 all who have sufficiently studied their structure and the 

 manner in which their functions are performed. 



But the place in the Animal Kingdom which is to be as- 

 signed to the sponges has been, and still is, a matter of de- 



