PORIFERA 65 



spore formation being exceptional. It has been esti- 

 mated that by self-division a Parameciunt may give rise 

 to 1,364,000 in forty-two days (Figs. 10, n). 



SERIES II. METAZOA 



The Metazoa include all those animals whose bodies 

 are multicellular, which reproduce by true eggs and 

 spermatozoa. This series includes eleven of the 

 branches of the animal kingdom. 



Branch II. PORIFERA 



The position of the sponges has been much dis- 

 puted. At first they were thought to be on the border 

 line between animals and plants, and were assigned by 

 some to the animals and by others to the vegetables. 

 Later, and up to very recent years, they were assigned 

 to the Protozoa. The discovery 



of their mode of reproduction 

 and development has determined 

 that they belong to the Metazoa. 



Simple sponges, like Grantia 

 (Fig. 13), are somewhat vase- 

 shaped in outline, and have a 

 single central cavity communi- 

 cating with the outside through 

 an opening called the osculum. 

 The wall of the body is pierced 

 by numerous fine canals which 

 communicate more or less di- FlG - X 3- -Diagram of a simple 



sponge: t, inhalant opening; 



rectly with the central cavity on o, exhaiant opening or oscu- 

 the one hand and with the exte- 

 rior on the other. There is no body cavity. The 

 body wall is composed of the skeleton, together with 

 the cellular elements forming the "flesh." The sur- 

 DODGE'S GEN. ZOOL. 5 



