134 I. IJIMA : HEXACTINELLIDA. I. 



and Dictyouine species/^' I cau not but maintain that the meshes 

 in question are all open and admit of a free passage of water 

 from the incurrent lacunfe into the interior of the chamber. 

 Only when the specimen is badly preserved or when the proto- 

 plasm of the choauocytes is insufficiently stained, is difficulty 

 experienced in deciding whether or not the mesh spaces are over- 

 spread and closed by a transparent membrane, but in successful 

 preparations the contour line of the reticular beams stands out 

 sharp and distinct against the perfectly empty meshes, so that 

 there can be no doubt whatever of the freely open nature of each 

 and every mesh in the reticular membrane. That this is not due 

 to the drastic effect of the preserving reagents, I have fully satis- 

 fied myself by repeatedly comparing the results of experiments 

 conducted according to different methods (see p. 34). 



I regard all the numerous meshes of the reticular membrane 

 as representing so many prosopyles. There exists among them 

 none that is particularly distinguished from the rest by a 

 specially large size or by a rounded shape. The above stands in 

 marked contrast to the condition we usually observe in other 

 sponges. As is well known, the prosopyles in certain forms 

 occur in tolerably large numbers to each chamber, but these 

 always break through the choanocyte epithelium in a scattered 

 distribution. Whereas, in the Hexactinellida they are to be con- 

 sidered as establishing themselves in all available interstices 

 between the individual choanocytes, converting the epithelium 

 into a veritable sieve-membrane. This state, in my opinion, 

 arises, because of the minimum development or, more probably, 

 of the utter non- development, of mesogloea in the parenchyme 



*Tlie reticular membraue of many of these species will be figured and remarked upon 

 iu future numbers of this series of Contributions. 



