CHAPTER VIII 
PHYLUM PORIFERA—SPONGES 
Class I. CALCAREA. 
Class 11. HEXACTINELLIDA. 
Class III. DEMOSPONGIA. 
SPONGES seem to have been the first animals to attain 
marked success in the formation of a “body.” For though 
their details are often complex, their essential structure is 
simpler than the average of any other class of Metazoa, and 
some of the simplest forms do not rise high above the level 
of the gastrula embryo. A “body” has been gained, but 
it shows relatively little division of labour or unified life ; it 
is a community of cells imperfectly integrated. The cells 
of the body show an arrangement in two distinct layers, 
which is one of the most essential characters of the 
Metazoa. There are no definite organs, and the tissues 
are, as it were, in the making. Sponges are passive, 
vegetative animals, and do not seem to have led on to 
anything higher; but they are successful in the struggle 
for existence, and are strong in numbers alike of species 
and of individuals. 
GENERAL CHARACTERS 
Sponges are diploblastic (two-layered) Metaszoa, the middle 
stratum of cells, the mesoglea, not attaining to the definiteness 
of a proper mesoderm. There is no celom or body cavity. 
The longitudinal axts of the body corresponds to that of the 
embryo; tn other words, the general symmetry of the 
gastrula ts retained. In these three characters the Sponges 
