PORIFERA. 153 



every direction through the sponge, and which are called " inhalant 

 canals," as it is through these that the water is conveyed to the 

 interior of the sponge. The " inhalant canals " ultimately open into 

 a second series of canals, which converge to form one or more large 

 tubes which open on the surface by a corresponding number of 

 large openings. These large tubes (fig. 47, ic) carry the water out of 

 the organism again, and they are hence called " exhalant canals " ; 

 while their surface-openings are known as the " oscula." The 

 " oscula," though capable of being temporarily closed, are perma- 

 nent, and are often placed on chimney-like elevations. If there 

 should be but one osculum, it is placed at the apex of the sponge 

 (fig. 47, o), while the pores occupy the general external surface. 

 What is commonly called a " sponge " may consist of only a single 

 excretory opening or " osculum," together with the " pores " belong- 

 ing to this (fig. 47) ; or it may consist of a larger or smaller number 

 of such " oscula," each with its proper complement of " pores." In 

 the latter case, each osculum, with its accompanying pores, consti- 

 tutes a " person," and the entire organism is known as a " sponge- 

 stock." 



In a living sponge, in its active condition, a circulation of water 

 is kept up throughout the organism by means of this system of canals. 

 The water is admitted from the exterior by the pores, and is driven 

 through the deeper parts of the sponge by means of flagellate cells, 

 which line a series of globular chambers developed between the 

 inhalant and the exhalant canals (fig. 47, ch). From these " flagellated 

 chambers " the water is driven into the exhalant canals, and finally 

 escapes again by the osculum or oscula. In this way, the separate 

 sponge-cells are enabled to carry on their nutritive and respiratory 

 processes. 



In a few sponges (the Myxospongioz of Haeckel) there is no 

 skeleton, and the above description would, therefore, fully express 

 the general structure of the organism. In the vast majority of 

 sponges, however, the soft cellular body is supported by more or 

 less extensively developed hard structures, which collectively consti- 

 tute the skeleton. The nature of the skeleton varies greatly in dif- 

 ferent forms, and these variations have been largely made use of in 

 the identification and classification of the Sponges. Speaking gen- 

 erally, the skeleton has the form of a more or less coherent frame- 

 work, composed either of horny fibres, or of needles of mineral mat- 

 ter, or of both these elements in combination. The different modi- 

 fications of the skeleton will be more particularly spoken of in deal- 

 ing with the different groups of sponges. It will be sufficient to 

 point out here that, apart from modifications in the form of the 

 skeletal elements, there are the following four principal types of 

 skeleton among the Sponges : — 



