40 



INVERTEBRATE ANIMALS. 



elucidation that their trae nature has been made out. Most 

 naturalists are now agreed as to the propriety of placing the 

 sponges in the animal kingdom, and they are generally re- 

 ferred to the Ehizopoda, though they are sometimes looked 

 upon as constituting a distinct and separate class of the I^o- 

 tozoa. The apparent complexity of structure which the 

 sponges exhibit is due to the fact that what we ordinarily 



f"ia. l.—a Siliceous shell of CoUofphaera ; b Tlialassiedlla, showing the radiating psead»- 

 podia and groups of siliceous spicula (after Muller). 



term a sponge is really a colony or aggregation of separate 

 masses of sarcode, greatly resembling Amoebm in structure, 

 and having the power of secreting a skeleton or supporting 

 framework common to the whole assemblage. Sponges, in 

 fact, may be defined as compound Mhizopoda, forming 

 masses which are traversed by canals opening on the surface, 

 and supported by a framework of horny fibres or of calcare- 

 ous or flinty needles. 



There are, then, two essential elements in the structure of 

 a sponge — namely, the saroode-bodies which constitute the 

 animal itself, and which are collectively termed the " sponge- 

 flesh," and the hard framework or " skeleton " upon which the 

 flesh is supported. To understand the nature of these fully, 

 we may take an ordinary horny sponge, such as we are con- 

 stantly in the habit of using. As we see the sponge in this 

 country, we are only acquainted with the skeleton, Avhich is 

 composed of an enormous number of horny fibres, all interlaced 

 and interwoven with one another, but leaving numerous holes 

 and canals between their bundles (Fig. 9, d). In its living 

 condition, however, the whole of this skeleton is covered in- 

 side and outside — saturated, in fact — with a kind of slimy ma- 

 terial very like white-of-egg to look at. This is the so-caUed 

 sponge-flesh, and, upon examining this with a microscope, it 

 is found to be composed of an enormous number of minute 



