CHAPTEE III. 



PORIFERA.* 



THE Porifera present a great variety of external form. They 

 may be cup-shaped, saucer-shaped, tubular, rod-shaped, foliaceous, 

 trumpet -shaped, fan-shaped, mushroom -shaped, lobed, digitate, 

 branched, or irregular, etc. (Figs. 60-62.) As a general rule, the 

 form is extremely variable even in the same species, and is therefore 

 of little use in identification. They are almost, if not quite, always 

 attached to foreign objects ; this may be effected by a broad basal 

 surface, or they may be stalked. In some cases they are rooted 

 in sand or in mud by basal processes or by special rooting spicules. 

 With the exception of the fresh-water Spongillidce, they are marine, 

 and are found at all depths. One family the Clionidce bore into 

 shells and stones. 



As the name implies, the surface of the body presents a large 

 number of pores, which are minute in size and inhalent in function. 

 These pores lead into a system of channels which, after permeating 

 almost the whole body, open to the exterior by one or more but 

 always a few larger exhalent openings called oscula. This system 

 of spaces connecting the inhalent pores with the exhalent oscula is 

 the canal system. Through it there passes maintained, as we shall 

 see, by ciliary action a continual stream of water, which enters by 

 the inhalent pores and passes out through the oscula. The sponge 

 is covered by an epithelial layer which we may call ectoderm; the 

 canal system is lined by an epithelium, which as we shall see is 

 usually partly ectoderm and partly endoderm ; but the main mass 

 of the body is formed of a soft tissue which we shall call mesodenn. 

 The mesoderm consists of a gelatinous basis (though no gelatine has 

 been detected in it), containing a protoplasmic network holding 



* For principal literature see classes and orders. E. Hanitsch, " Revision of 

 the generic nomenclature and classification in Bowerbank's 'British Spongiadae.'" 

 Proceedings and Transactions of the Liverpool Biol. Soc., vol. 8, 1893, p. 173. 



