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, x ' \ 



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 



mark othprs a^ain form Fla 26 - The Bread-crumb Sponge 

 marK otners agair . (Ha i ichondria panicea} ( liat ural size). 



much larger arid more COm- 0) Oscula ; P) inhalent pores. 



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 In the simplest sponge- 

 Structure. body, the cells are so 

 Ascon Type. arrange d 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) 

 of Hydra ; the inner cells correspond 



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



. 



Ascon type ; e, ectoderm ; c, being all very definitely flagellate, 

 collar ceils ; i, inhaient an( j a i so t h e ce u s bearing the flagella 



pores ; o, osculum. , v ,.,,, P . 



B, one collar ceil, enlarged. na ve a 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 



