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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. 
Snbkingdom II. —Porifera. 
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, but by few naturalists are still so considered. 
FiG.lS9.-Hypothetic.nl Section of a Sponge: a, superficial layer; 5, inhalant pores; 
c, ciliated chambers; d, exhalaut aperture, or osculum; e, deeper substance of 
the Sponge. 
