PROTOZOA: SPONGIDA. 8/ 



" efferent " canals by which it reaches the " oscula " and is 

 finally expelled from the body. These processes are regularly 

 performed, and their mechanism was long a subject of spec- 

 ulation. It is now known, however, that beneath the super- 

 ficial layer or " dermal membrane " of the sponge there exist 

 chambers lined with sponge -particles which are provided 

 with vibratile filaments or flagella (fig. 25, c c). The pores 

 open into these chambers, and from them proceed the incur- 

 rent canals, each being dilated at its commencement into a sac, 

 which is also lined with flagellate sponge-particles. By the 

 vibratile action of these cilia, currents of water are caused to 

 set in by the pores ; and as out-going currents proceed from 

 the oscula, a constant circulation of fresh yvater is maintained 

 through the jentire^ sponge. In this way each individual sponge- 

 particle is enabled to obtain nutriment ; the process being at 

 the same time not improbably a rudimentary form of respira- 

 tion. The chambers or sacs lined with flagellate sarcoids have 

 b'een shown by Mr Carter to be, as previously pointed out, 

 the essential element in the organisation of the fresh-water and 

 marine sponges, and to be the fundamental expression of the 

 alimentary system. 



In a few sponges (the Myxospongia of Haeckel), as in the 

 genus Halisarca, there is no skeleton, and the organism con- 

 sists of an aggregate of masses of sarcode, permeated by 

 branched canals, which are everywhere inflated into cham- 

 bers lined by flagellate sponge-particles or sarcoids. As a 

 general rule, however, the soft protoplasmic aggregate which 

 constitutes the living animal of the sponge is supported by 

 more or less extensively-developed hard structures, which col- 

 lectively form the skeleton. The nature of the skeleton varies 

 greatly in different forms, and the variations are of great im- 

 portance in the identification and classification of the sponges. 

 In the so-called " horny " sponges (the Keratosa of Bowerbank) 

 the skeleton (fig. 26) is composed of numerous fibres of a horny 

 substance (" keratode ") interlaced to form a matted network. 

 In the sponges of commerce (Spongid) the skeleton is simply 

 composed of these reticulated horny fibres ; but in most of 

 the " horny " sponges (such as Halichondria, Spongilla, &c.), 

 we find in addition numerous siliceous bodies which partly 

 strengthen the horny fibres, and are partly scattered through 

 the sarcode. These so-called "spicules" (figs. 26 and 27) 

 are of very varied shapes, and of microscopic dimensions, and 

 their form is often characteristic of the particular sponge in 

 which they occur. 



The horny fibre of the skeleton of the keratose sponges is hollow ; and 



