CHAPTER TIL 



THE PORIFERA AND THE CCELENTERATA. 



1. The Porifera or Sroxgida. — It lias been seen that, 

 in the Protozoa, the germ underg'oes no process of division 

 anak)gous 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 

 collular embrj^o. 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 histoloffical 

 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 alwavs 

 occurs ; and, very generally, though bv 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 betw^een the vegetable and the animal king- 

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

 all who have sufTiciently studied their structure and the 

 manner in which their functions are performed. 



]5ut the place in the Aiiimal Kinirdom which is to be as- 

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



