CHAPTER VIII. 



PHYLUM PORIFERA SPONGES. 



Class I. CALCAREA. 



Class II. HEXACTIXELLIDA. 



Class III. DEMOSPOXGI.^F,. 



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] Metazoa, the middle 

 stratum of cells, the mesoglcea, not attaining to the definiteness 

 of a proper mesoderm. There is no ccelom or body cavity. 

 The longitudinal axis of the body corresponds to that of the 

 embryo ; in other words, the general symmetry of the 

 gastrula is retained, hi these three characters the Sponges 



