PROTOZOA : SPONGIDA. 55 



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 of horny reticulated 

 fibres which interlace in every direction, and are pierced by 

 numerous apertures, the whole surrounded externally 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 ' exhalent apertures ; ' whilst others, which occur in much 

 greater numbers, are greatly smaller in size and are termed 



* pores' or 'inhalent apertures.' The 'sponge-flesh' which 

 invests the entire skeleton is found upon a microsopical 

 examination to be entirely composed of an aggregation of 

 rounded amcebiform bodies the so-called ' sponge-particles ' 

 or 'sarcoids' (fig. 2 c, d, e). Some of these are ciliated ; 

 whilst others are capable of emitting pseudopodia from all 

 parts of their surface, and are provided with nuclei, thus 

 coming closely to resemble so many AmodbcB. Regarding .the 

 skeleton as something superadded, we may, in fact, look upon 

 a sponge as being essentially nothing more than an aggrega- 

 tion of Amcehce, 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. 



In a living sponge a constant circulation of water is main- 

 tained by means of an aquiferous system (fig. 9), which is 

 constituted by the oscula and pores already alluded to and 

 by a system of canals, excavated in the substance of the 

 sponge, and uniting the two sets of apertures. The water 

 passes in by the ' pores ' or inhalent apertures, and is con- 

 veyed by a series of canals the 'incurrent' or 'afferent' 

 canals to a second series of tubes the 'excurrent' or 

 '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 specu- 

 lation. It is now known, however, that beneath the superfi- 

 cial layer or 'dermal membrane' of the sponge there exist 

 chambers lined with sponge-particles which are provided, 



