POKIFERA (sponges). 43 



CHAPTEE IV. 

 PORIFEEA (SPONGES). 



Originally regarded as being of a vegetable nature, the Sponges 

 are now universally admitted to be animals; and are by many 

 naturalists associated with the Sea-anemones and Corals (Vtelenter- 

 ata). They have, however, many remarkable points of relationship 

 to the typical Protozoa, and particularly to the Flagellated Infusoria. 

 Upon the whole, therefore, it may be as well to treat them here as 

 a special group, intermediate between the Protozoa and Calenterata, 

 under the name of Porifvra. 



The Sponges may be defined as mnlticAlular protoplasmic masses, 

 supported in roost cases hy a framework of horny Jihres or of cal- 

 careous or flinty needles, and traversed by canals vihich open on the 

 surface, and are dilated at intervals into chambers lined by flagellate 

 cells. 



In the first jilace, then, a Sponge differs from the typical Protozoa, 

 and agi-ees with all the higher animals {Mctazoa), in the fact that 

 the body is multicellular, being composed of a vast number of 

 separate protoplasmic units, which may be termed " sponge-par- 

 ticles " or " sarcoids," and each of which may be regarded as being 

 a single cell. In the second place, this protoplasmic aggregate is 

 almost always, though not invariably, stiengtheued and endowed 

 with a definite form, by the presence in it of a " skeleton," consist- 

 ing of horny fibres, or of " spicules " of hme or silica. In the third 

 place, the entire organism is traversed by a complicated " canal- 

 system," which opens on the surface by two sets of apertures 

 ("pores" and "oscula"), and the branches of which are dilated at 

 intervals into " ciliated chambers." There is no distinct mouth, 

 no digestive system, no circulatory or definite nervous organs, and 

 no body-cavity ; but we must consider in greater detail the char- 

 acters and structure of the "sponge-flesh," the "skeleton," and the 

 " canal -system." 



If we take the ordinary horny sponges of commerce, what we 

 call the " sponge " is, cjf course, nothing but the skelttoii of the 



