PROTOZOA: SPONGIDA. 83 



cesses of firm sarcode, which have been regarded as of a chit- 

 inous nature. Acanthocystis is, like the preceding, a fresh-water 

 form, but it possesses long radiating siliceous spines; while 

 Clathrulina has the body enclosed in a regular fenestrated sili- 

 ceous test, which is supported upon a siliceous peduncle. 



CHAPTER V.- 



SPONGIDA. 



ORDER V. SPONGIDA or PORIFERA. The true nature of sponges 

 has long been a matter of dispute, but they are now universally 

 referred to the animal kingdom, their precise systematic position 

 being still a matter of dispute.* 



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. 26) 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 often 

 strengthened by sand-grains, or by spicula of flint which also 

 occur less abundantly in the sponge-flesh. These latter must 

 not, however, be confounded with the skeleton of the typical 

 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 



* High authorities consider the sponges as a division of the Ccelenterata, 

 or as forming, under the name of Porifera, a division of the " Zoophyta " 

 coequal with the Coelenterata. This view, however, is based upon the in- 

 terpretation of sponge-structure adopted by Hseckel, and the sponges will 

 be here regarded as referable to the sub-kingdom Protozoa. 



