STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 527 



a very aberrant one, and the new genus Rid lei a forms a 

 connecting link between it and the other members of its sub- 

 family. 



Probably Quasillina is to be regarded as a more highly 

 modified form than Rid lei a. I have already pointed out that 

 the arrangement of the fibrous tissue in definite bands indicates 

 a higher degree of modification than that which exists in 

 Ridleia, where it forms more or less continuous sheaths, and 

 the occurrence of the stylodragmata appears to me to be an 

 adaptive modification resulting from the lacunar character of 

 the canal system, and the general delicacy of the choanosome. 

 Stylodragmata occur, so far as I am aware, in no other members 

 of the group. 



Professor Schulze, in his 'Report on the "Challenger" Hex- 

 actinellida/ and elsewhere, has expressed the opinion that the 

 polyactinal type of spicule is the primitive form from which 

 the monactinal type has been derived by abortion of the rays. 

 Although in a previous paper 1 I upheld, for reasons therein 

 given, the contrary hypothesis, I am now inclined to regard 

 Professor Schulze's view as the more correct one. 



It is, I believe, generally admitted that the swollen base, or 

 head, of a typical Suberitid spicule, together with the corre- 

 sponding enlargement of the axial thread, indicates the position 

 where other rays were at one time united with that one which 

 now alone remains. In the typical Suberitidse, then, and in 

 Ridleia, all rays but one have disappeared, but their former 

 presence is still indicated by the head of the tylostylote spicules. 

 In Quasillina the spicules are still more modified, and even 

 the head has, in most cases, disappeared. 



Judging, then, from the condition of the fibrous tissue 

 and of the spicules, we should expect the canal system of 

 Quasillina to be less primitive that that of Ridleia, and, 

 in general, the lacunar type of canal system, as it occurs in 

 the Monaxonida, with chambers opening directly into wide 



1 Dendy and Ridley, "On Proteleia Sollasi," 'Ann. and Mag. Nat. 

 Hist./ ser. 5, vol. xviii, p. 153. 



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