REPOKT ON THE MONAXONIDA. Ivii 



all of the greatest importance to the sponge, and subject to great modification with every 

 change in external conditions. Of this we have abundant proof ; we have seen already 

 how sponges in warm climates tend to develop much spongin, while the megasclera 

 gradually become fewer in number and less in size. We have also shown that the 

 arrangement of the pores is largely dependent on the arrangement of the dermal 

 skeleton and varies with this, and so forth. But the form of the microsclera does not 

 appear to be dependent upon external conditions or upon any other part of the organisa- 

 tion, and hence we find in them great constancy. Let us take, for example, the large 

 family of the Desmacidonidse. In this famUy we find megasclera of several distinct types, 

 and their arrangement in the skeleton varies much ; the arrangement of the pores 

 varies enormously, and the external form if possible varies even more. But throughout 

 the whole family we find one and the same fundamental form of microsclera (chelse), 

 commonly associated with others. These chelse are such remarkable and complex forms 

 that we cannot possibly imagine that they have arisen independently in the diS"erent sub- 

 families and genera, and hence we are obliged to place all these together and separate 

 them from the remainder of the Monaxonida, none of which possess chelae. 



To sum up briefly, the result of our investigation is as follows : — (1) AVe must 

 always endeavour to classify by an assemblage of characters ; (2) of individual 

 characters the form of the microsclera (when present) is the most important, and this 

 owing to the fact that these are subject to less modification than other parts of the 

 organism. 



11. The Classification of the Monaxonida here adopted. 



Having thus investigated the data of classification and arrived at certain conclusions 

 with regard thereto, we must now set forth the results to which these conclusions have 

 led us in classifying the Challenger collection, and then discuss the mutual relations of 

 the various subdivisions. 



In the following scheme of classification we shall mention only those genera which 

 are present in the collection, as the time at our disposal is not sufficient to enable us to 

 give a complete resume of this very intricate subject. The Challenger collection is, how- 

 ever, very fairly complete as regards genera, and quite sufiiciently so to justify this 

 proceeding, and although other genera are not here mentioned it must not therefore be 

 thought that we have ignored them in considering the question. The present scheme is 

 based upon the schemes of previous writers, which have been more or less assimilated and 

 modified in accordance with our increase of knowledge. 



For the sake of brevity we have also decided to omit in this place all diagnoses ; 

 these will be found given fully in the Description of Genera and Species, to which the 

 reader is referred. 



(ZOOL. CHALL. EXP. — PART LIX. — 1887.) Nnn h 



