130 SPONGES 



belongs to the choanosome and cannot therefore be compared with 

 a subdermal cavity, it would appear that in other cases a cortex 

 may be developed simply as a great thickening of the dermal mem- 

 brane, in which case the subcortical crypts may belong to the 

 ectosome and represent subdermal cavities. A cortex is, in fact, 

 a structure which can develop in different ways and may not be 

 homologous in different sponges. The term " subcortical crypt " is 

 to be understood therefore in a descriptive rather than in a 

 morphological sense. 



The following table may serve to indicate the homologies of the 

 incurrent system in three typical cases : 



& 



o 



Each of the above types of the incurrent system may be combined 

 with different forms of the canal system considered as a whole, 

 especially as regards the relations of the chambers to the excurrent 

 and incurrent canals. As is plain from what has already been stated 

 with regard to the development from a Khagon, the canal system 

 of Demospongiae always conforms to what has been termed above 

 the third type ; but within the limits of this type of structure, it 

 may be either eurypylous, aphodal (Fig. 88), or diplodal (Fig. 89). 

 Hence the canal system as a whole is liable to very great structural 

 variations in the Demospongiae. 



Skeleton. The skeleton of the Demospongiae exhibits variations of 

 so divergent a character that it is not possible to discuss it in general 

 terms. We have to consider first those forms in which the skeleton 

 is composed of siliceous spicules, some or all of which are of tetraxon 

 type (Tetraxonida); secondly, those which always possess siliceous 

 spicules of monaxon form and never tetraxon (Monaxonida) ; and 

 thirdly, those in which proper spicules i.e. spicules secreted by 

 the sponge are absent and the supporting framework is made up 

 of spongin fibres alone (Keratosa). 



(a) Tetraxonida. The siliceous spicules which compose the 

 skeleton of the Tetraxonida are divisible into megascleres and 

 microscleres two categories which in the order Tetractinellida are 

 sharply distinct from one another, differing not only in size and 



