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COMPARATIVE ZOOLOGY. 



SERIES IL— METAZOA. 

 The Metazoa include all those animals which reproduce 

 by true eggs and spermatozoa, whose germ develops a 

 blastoderm, and which have cellular tissues. There are 

 seven subkingdoms. 



Subkingdom I. — Spongida. 

 The position of the Sponges has been much disputed. 

 At first they were thought to be on the border-line be- 

 tween 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 Proto- 

 zoa. The discovery of their mode of reproduction and 

 development has determined that they belong to the 

 Metazoa. 



The Sponges are formed of an aggregate of membrane- 

 less amoeboid or ciliated cells. They usually have a skele- 

 ton, which may be calcareous, horny, or siliceous. They 

 have a central cavity, with numerous incurrent orifices 

 and one excurrent opening. They reproduce by true 

 eggs, as well as by budding and fission. 



The cells of the Sponge are relatively independent, 

 whence they have been regarded as colonies of amoeboid 

 animals, and by some naturalists are still so considered. 



Fig. 189.— Hypothetical Section of a Sponge: a, superficial layer; 2>, inhalant pores; 

 c, ciliated chambers; d, exhalant aperture, or osculum ; e, deeper substance of 

 the Sponge. 



