CALCAREOUS SPONGES. , 709 



PRINCIPLES OP CLASSIFICATION. 



It appears to us that the chief point to be borne in mind in 

 attempting to arrive at any natural system of classification is the 

 importance of utilising as many characters as possible. A classifi- 

 cation based upon a small number of characters mvist necessarily 

 be arbitrary and artificial, and characters which are of great 

 importance in some cases may be of comparatively little use in 

 others. There can be no doubt that there are certain significant 

 characters which do indicate genetic relationships, but these 

 characters are by no means the same in all cases, and they have 

 to be carefully sought for and distinguished amidst a host of less 

 important features. Sometimes it is the canal system that affords 

 the best clue, sometimes the ari\'ingement of the skeleton, some- 

 times the form of the spicules, and sometimes even the position of 

 the nucleus of the collared cells. There mvist also be remembered 

 the undoubted fact that the j^henomenon of convergence has 

 played a large jaart in the evolution of the Calcarea, and has led 

 in many cases to totally deceptive resemblances, as, for example, 

 between the genera L&ucetta and Leucandra. 



It seems likely, however, that the collared cell, or choanocyte, 

 which itself is by far the most characteristic histological con- 

 stituent of the sponge organisation, may ultimately prove, as 

 suggested by Bidder [1898], to afford a means of dividing the 

 whole of the Calcarea into two main branches, one having the 

 nuclei of these cells placed basally, and the other having them 

 apical. Indeed, the acceptance of this principle, if only in a 

 tentative manner, constitutes the chief difference between our 

 present views on the subject and those which we previously held ; 

 but in the present state of our knowledge it is a, principle which 

 must not be pushed too far, and we have only been able to make 

 vise of it as subsidiary to more easily determined characters. The 

 acceptance of this principle, however, necessitates the wide sepa- 

 ration of the Lelapiidje from the other Pharetronid sponges with 

 which they have hitherto been associated. 



We shall discuss this question in some detail later on, but it 

 may serve a useful purpose if we give at once a list of all the 

 species of Calcarea in which we have been able to determine the 

 position of the nucleus, and the results of our determination. 

 This list includes no less than 75 species, as follows : — 



Nuclei apical. Nuclei basal. 



Family HoMOCCELiDiE. 



Leucosolenia hella Row. Leucosolenia falcata Haeckel. 



Leucosolenia compUcata Montagu, Leucosolenia stolonifer Dendy. 

 fide Minchin. Leucosolenia ventricosa Carter. 



Leucosolenia lucasi Dendy. Leucosolenia gardineri Dendy. 



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