CHAPTER VIII. 



PORIFERA— SPONGES. 



A. Calcarea (Calcispongias). 



-r, , T ,-, . t Hexactinellida. 



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 general 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. 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 1 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. In these three characters the Sponges agree zvith 

 the Ccelentera, and differ from higher (triploblastic and 

 cxlomate) Metazoa. 



The body varies greatly in shape, even within the same 



