60 MANUAL OF ZOOLOGY. 



Thalassicolla differs little from either of the above in funda- 

 mental structure, but it contains a number of compound 

 siliceous spicules embedded in its ectosarc (fig. 7, b). 



CHAPTER V. 



SPONGIDA. 



THE true nature of sponges has long been a matter of dispute, 

 but they are now almost universally referred to the animal 

 kingdom, and placed either in or near the Rhizopoda. Some 

 observers still maintain the vegetable nature of sponges, but 

 this opinion has no real grounds for its support, and is chiefly 

 founded upon loose analogies, and upon a certain similarity in 

 outward form. 



The Spongida may be defined as " sar code-bodies, destitute of 

 a mouth, and united into a composite mass, which is traversed by 

 canals opening on the surface, and is almost always supported 

 by a framework of horny fibres, or of siliceous or calcareous 

 spicula. " ( Allman. ) 



From the above definition it will be seen that a sponge 

 is composed essentially of two elements a soft, gelatinous, 

 investing " flesh," and an internal supporting framework or 

 " skeleton." 



Taking an ordinary horny sponge as the type of the order, 

 we find it to be composed of a skeleton (fig. 9, d) of horny 

 reticulated fibres which interlace in every direction, and are 

 pierced by numerous apertures, the whole surrounded exter- 

 nally and internally by a gelatinous glairy substance, like white 

 of egg, the so-called " sponge-flesh." The horny skeleton is 

 composed of a substance called "keratode," and is usually 

 strengthened by spicula of lime or flint, which also occur less 

 abundantly in the sponge-flesh. These must not, however, be 

 confounded with the skeleton of the true calcareous or siliceous 

 sponges in which the keratode is wanting. Of the apertures 

 which penetrate the substance of the sponge in every direction, 

 some are large crateriform openings, and are termed " oscules," 

 or " exhalant apertures ; " whilst others, which occur in much 

 greater numbers, are greatly smaller in size, and are termed 

 "pores," or "inhalant apertures." Both the oscula and pores 

 can be closed at the will of the animal ; but the oscula are 

 permanent apertures, whereas the pores are not constant, but 



