152 PROTOZOA. 



CLASS PORIFERA (poros, canal; pkero, I bear). 



The Porifera include the Sponges, and are not to be regarded as any more 

 highly organized than the Rhizopoda. A sponge consists of a congeries of horny 

 filaments, interlaced in every direction so as to form an intricate network of inter- 

 communicating cells. Imbedded in these filaments, in the majority of sponges, are 

 a number of minute needle-shaped, or forked, or radiated silicious, or calcareous 

 particles of various forms, called spiculse. The spiculse may be acicular and pointed 

 at both ends, or have a small knob at one end, while the opposite end is pointed ; 

 or one end may be a fork, with two or three prongs. The horny filaments, with 

 their contained spiculse, constitute the skeleton which supports the living sponge. 

 The living sponge consists of a mere coating of gelatinous matter spread over all 

 the filaments, of the consistence of the white of an egg, which runs freely away 

 from the skeleton or framework of the sponge when taken out of the water. 

 Under the microscope this gelatinous matter is found to consist of an aggregation 

 of sarcode cells, and each cell appears to possess an independent existence ; and even 

 when detached from its fellows it has the power to move by the extension of its 

 substance in various directions. In a living sponge there is an infinite number of 

 minute holes, and a lesser number of larger openings. The water is imbibed 

 through the smaller pores, and thrown out from the larger ones. The circula- 

 tion results from the action of cilia, in much the same way motion is effected l>y 

 the Rhizopoda. 



Sponges attach themselves to all kinds of objects, whether fixed or floating. 

 Some cover rocks and shells with a spongy incrustation ; others hang from floating 

 sea-weeds, and others shoot up branched stems, or a massive, globular framework. 

 The Cliona is a boring sponge, that imbeds itself in shells or other calcareous sub- 

 stances. Sponges of the same species assume very different forms. In fact, there 

 are no animals in which the variations are as great in a single species. They 

 attain their greatest development in tropical seas, but occur in the most northern 

 latitudes. 



The genera and species of living sponges are largely founded upon the frame- 

 work and spiculse, and of course the same characters are sought in fossil sponges 

 for the purpose of classification. Among the Palseozoic sponges, form is of much 

 more importance than it is among living sponges, as we may believe, because 

 we find so many specimens of the same form and size in a given species, not only 

 at one locality, but at distant places, even hundreds or thousands of miles apart, 

 in the same Group of rocks ; as, for instance, Astylospongia prcemorsa, on the Island 

 of Gottland, in the Baltic Sea, and in Tennessee and Indiana. When Silurian 

 sponges are silicified, the surface is generally ver^ poorly preserved, and the spiculse 

 perfectly preserved ; but calcareous and unsilicified specimens of the same species 

 will show a well-preserved exterior and no spiculse. It is therefore impossible to 

 determine whether the sponge in its living state had calcareous or silicified spiculse. 

 In the fossilization of sponges and other bodies, and even long after fossilization has 

 taken place, silica will be taken up, and lime will be deposited in its place in some 

 waters ; while in other waters lime will be taken up, and silica will be deposited 

 in its stead. An original calcareous sponge, when converted into a silicious fossil, 

 will preserve the spiculse ; but if a sponge bears silicious spiculse, and is converted 



