CHAP. IV PORIFERA 53 



or it may form a branched finger-like mass, as in the common 

 Chalina oculata, often thrown up on the beach after storms ; 

 or it may form encrusting 

 masses with many oscula, 

 each raised on a little pro- 

 jecting crater-like process, as 

 in the crumb-of-bread sponge, 

 Halichondria (Fig. 26), which 

 is common on rocks and 

 weeds even above low tide 



tnarlr • nfhprs acrain form Fk- 26.— The Biead-onimb Sponge 



marK, Otners again torm ^ffalichondriapanicea) {na.tnT!d size). 



much larger and more com- „^ ^^^^^ . ^^ i„^^i^„^ ^^^^^ 



plex bodies, as in the Bath 



Sponges, which are found chiefly in the Mediterranean Sea, 



and also off the West Indies. 



In all cases the sponge-body is found to have a certain 

 general type of structure, and a 

 similar differentiation of tissues. 

 Microscopic I" ^^^ simplest sponge- 

 Structure, body, the cells are so 

 Ascon Type, arranged that they form 

 two distinct layers, surrounding a 

 single cavity, which opens to the 

 exterior by the osculum; but the 

 osculum, it must be remembered, is 

 not a mouth, but an aperture for the 

 casting out of the water taken in 

 through the pores all over the walls. 

 The outer layer of cells corre- 

 sponds to the skin cells (ectoderm) 

 Fig. 27. of Hydra ; the inner cells correspond 



A, Diagrammatic representation to the endodcrm of Hydra, but the 



ttouVT^^ofrpi: latter are peculiar in sponges in 

 Ascon type ; e, ectoderm ; e, being all Very definitely flagellate, 

 collar cells ; i, inhaient and also the cells bearing the flagella 



pores; o, osculum. , -,. t..i ■ ,. , 



B. one collar cell, enlarged. ^^"^ * peculiar little projecting trans- 



parent tube round the base of each 

 flagellum, whence they are called " collar cells." The whole 

 central cavity in these simplest forms is lined with collar cells ; 

 between the ectoderm and endoderm is a gelatinous layer, 

 the mesogloea; and embedded in this mesogloea are found the 



