70 MANUAL OF ZOOLOGY. 



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 13, 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 often 

 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 

 can be formed afresh whenever and wherever required. The 

 " sponge-flesh," which invests the entire skeleton, is found 

 upon a microscopical examination to be composed of an aggre- 

 gation of rounded amcebiform bodies the so-called " sponge- 

 particles " or " sarcoids " (fig. 3, c, d, e). Some of these are 

 ciliated ; whilst all are capable of emitting pseudopodia 

 from all parts of their surface, and are provided with nuclei, 

 thus coming closely to resemble so many Amoeba. Regard- 

 ing the skeleton as something superadded, we may, in fact, 

 look upon a sponge as being essentially nothing more than an 

 aggregation of Amcebcz, since each " sarcoid " is capable of 

 procuring and assimilating food for itself in a manner strictly 

 analogous to what we have seen in the Amoeba. This view 

 becomes still more easily comprehensible when we consider 

 the simplest condition in which a sponge occurs in nature (as 

 exemplified, for instance, in certain of the Calcispongice) ; the 

 condition, namely, in which the entire sponge consists of a 

 colony of amcebiform sarcoids, secreting a common skeleton, 

 but provided with only a single " osculum," and a greater or 



