SPONGIDA. 



133 



' 



are called " oscula," or " exhalant apertures." There may be 

 only a single osculum, or many may be present. The other 

 set consists of very much smaller openings, which are always 

 very numerous, and which are termed the "pores," or 

 " inhalant apertures." The pores and oscula are connected 

 by a system of canals excavated in the substance of the 

 sponge, and a constant circulation of water can be kept up 

 through the whole mass, the former serving for the incoming 

 currents, the latter for the outgoing. 



The Sponges are by far the largest of the Protozoa, and, as 

 above defined, they consist of a soft basis of living proto- 

 plasm, which, with hardly an exception, is supported by 

 certain skeletal structures, which vary in composition and 

 arrangement, and are more or less capable of being preserved 

 in a fossil condition. As the soft parts of the Sponges seem 

 to be essentially identical, and as it is only the supporting 

 framework or skeleton which is capable of undergoing petri- 

 faction, we may divide Sponges according to the nature of 

 their hard parts into the following three groups : 



1. THE HORNY SPONGES (Keratoda), in which the skeleton 

 is composed of a substance allied to horn, and consists of 

 innumerable fibres matted and 

 felted together, so as to give 

 rise to a very variably-shaped 

 mass. The fibres may be solid 

 or hollow, and the skeleton may 

 consist wholly of these, or may 

 be more or less extensively 

 strengthened by means of vari- 

 ously-shaped microscopic spic- 

 ules of flint (fig. 32). The 

 horny framework of Sponges 

 such as these is obviously in- 

 capable of preservation in the 



. Fig. 32. Fragment of the skeleton of 



lOSSll Condition ; UnleSS We SUp- a horny sponge (after Bowerbank), greatly 



pose (what has not been proved Sj^l owing ta 

 to occur) that it may be re- 

 placed, during the process of petrifaction, by flint or car- 

 bonate of lime ; while the spicules which are often present, 



