PROTOZOA : SPONGIDA. 6 1 



can be formed afresh whenever and wherever required. The 

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

 upon a microscopical 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 pseu- 

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

 nuclei, thus coming closely to resemble so many Amoeba. Re- 

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

 look upon a sponge as being essentially nothing more than an 

 aggregation of Amoeba, 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 

 less number of inhalant " pores." There are, in fact, many 

 who hold that the more complex sponges are merely produced 

 by the aggregation together of a number of these simpler 

 colonies. 



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

 tained by means of an aquiferous system (fig. 8), 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 inhalant 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 

 with vibratile filaments or cilia (fig. 8, c, c). The pores open 

 into these chambers, and from them proceed the incurrent 

 canals, each being dilated at its commencement into a sac, 

 which is also lined with ciliated sponge-particles. By the 

 vibratile action of these cilia, currents of water are caused to 

 set in by the pores ; and as out-going currents proceed from 

 the oscula, a constant circulation of fresh water is maintained 

 through the entire sponge. In this way each individual 



