40 



lumen of the choanocytal collars, and in this case they would 

 be very difficult to distinguish. It is also a curious fact that I 

 have never yet seen the flagella of the concrescent choanocytes, 

 though I have never failed to find them in the case of choano- 

 cytes which are not concrescent. It might be explained on the 

 supposition that the flagella are retracted in the former case ; but 

 that naturally leads to the iuquiry as to vv^hy they are not re- 

 tracted in the latter " 



On page ei the author dwells again on the systematic value 

 of the »concrescence of the choanocytes", pointing out thatit»is 

 possible that the concrescence of the choanocytes vyhich occurs in 

 all the higher Tetractinellids may eventually serve to distinguish 

 them from the Monaxons proper. Those Monaxons vs^hich have 

 descended from the various families of the Tetractinellida might 

 then be included by some such definition as the following: — 

 Tetractinellida, Demosponges characterised by desmas, triaenes, 

 or tetraxons, and where these are absent by the concrescence of 

 the choanocytes. 



But at present our knowledge of the minute characters of the 

 Monaxons is not complete enough to enable us to judge of the 

 value of this possibly distinctive character; in such Suberites as 

 I have examined there is no concrescence and I hold this provi- 

 sionally as completely separating this family from the Tetractinellids 

 and as uniting it with the Monaxonids; so too there is no con- 

 crescence in the Tethyidae, which must therefore also beassigned 

 to the Monaxonids ; on the other hand , in the purely Monaxonid 

 Sponge Amphius, which on morphological grounds we conjectu- 

 rally derive from the Stellettidae , concrescence has been observed, 

 and there would thus appear to be good reason for assigning it 

 to the Tetractinellida as a degenerate or simplified form. The 

 characters of the choanocytes in Placospongia have not yet been 

 made out, and thus although the presence of the sterraster in 

 this Sponge would suggest its afifinities with the Sterrastrosa , yet 

 till we know whether the choanocytes are concrescent or not it 

 may be as well to suspend judgment. For the present, therefore, 



