vin DEMOSPONGIAE 211 



others have a simple dermal membrane {Halichondria panicea, 

 Tetilla pedifera) and various others are provided with a cortex, 

 either of simple structure or showing elaboration in one or more 

 particulars. Thus a protective armature of special spicules may 

 be present in the cortex, e.g. in Geodia, or to a less extent in 

 Tethya, or there may be an abundance of contractile elements, 

 and these may be arranged in very definite ways, forming valve- 

 like apparatus that will respond to stimuli. 



Everywhere among sponges the goal of the skeleton appears 

 to have been coherence. We have seen how in Calcarea and 

 in Hexactinellida this has been attained by the secretion 

 around the separate elements of a continuous mineral sheath, 

 calcareous in the one case and siliceous in the other. Here we 

 had an excellent instance of the attainment of one end by similar 

 means in two different groups, after their separation from the 

 common stock, and therefore independently. In Demospongiae, 

 on the other hand, the same end coherence has been secured 

 by two new methods, each distinct from the former : first 

 the spicules may be united in strands by an organic deposit, 

 spongin ; secondly, the spicules may assume irregular shapes and 

 interlock closely with one another, forming dense and stout 

 skeletons. The latter method is that characteristic of the 

 Lithistid Tetractinellida. 



Classification. It is not of great moment which scheme of 

 classification we maintain, seeing that all hitherto proposed are 

 confessedly more or less artificial, and sufficient data for framing 

 a natural one are not yet forthcoming. For convenience, we 

 accept three subdivisions and define them thus : 



I. Tetractinellida. Demospongiae possessing tetraxon or triaene 

 spicules or Lithistid desmas. 

 II. Monaxonida. Demospongiae possessing monaxon but not tetraxon 

 spicules. 

 III. Ceratosa. Demospongiae in which the main skeleton is formed of 

 fibres of spongin. The fibres may have a core of sand-grains or of 

 foreign spicules, but not of spicules proper to the sponge. 



But at the same time we admit that some of the Ceratosa are 

 probably descended from some of the families of Monaxonida, so 

 that we should perhaps be justified in separating these families 

 of Monaxonida from the rest, and associating them with the 

 allied families of Ceratosa a method of classification due to 



