88 PORIFERA. 



oceans, are more numerous near the land. The HexacMnellida are 

 mainly deep-water forms, the characteristic depth being between 

 200 and 1000 fathoms. For the Tetractinettida the characteristic 

 depth is 50 to 200 fathoms, though they are also found in shallow 

 water. The Calcarea and Ceratosa are mainly shallow-water forms 

 (to a depth of 200 fathoms). The Monaxonida, though character- 

 istically inhabitants of shallow water, are found in considerable 

 numbers at all depths. The deep-sea Monaxonida are distinguished 

 by their symmetrical and definite shapes. 



All spongologists agree as to the immense difficulty of classifying 

 sponges. Not only are the boundaries of the great groups difficult 

 to lay down, but the limits of species, and even of genera, often 

 defy definition. Almost all characters are highly variable, and the 

 number of intermediate forms and of collateral affinities is immense. 

 On the whole we may distinguish three main types, which we shall 

 exalt to the dignity of classes not because they deserve that rank, 

 for they do not; but because the term fits in more conveniently 

 with the terminology generally used for the group. The three 

 classes are as follows : 



Calcarea. With calcareous spicules and large choanocytes. 

 Order 1. Calcarea. 



Triaxonia. With triaxonic (sex-radiate) spicules and large flagel- 

 lated chambers. 



Order 2. Hexadinellida. 



3. Hexaeeratina. 



Demospongise. Without triaxonic spicules ; with small choano- 

 cytes and ciliated chambers ; skeleton of silicious spicules or spongin, 

 or both combined. 



Order 4. Tetradinellida. 

 5. Carnosa. 

 6. Monaxonida. 

 7. Ceratina. 



While these groups stand out fairly sharply, their division into 

 sup-groups, or orders, as we must call them, is fraught with some 

 difficulty. This is particularly the case with the Demospongice, 

 which include the great majority of living sponges. 



The sub-divisions of the Demospongiae must be regarded as entirely artificial, 

 and only to be established for the convenience of the student. The four orders 

 into which we have divided the class are by no means sharply marked off from 

 one another, nor do they form a single series, but rather three or four series 

 running parallel to one another, and connected together at several points. In 



