REPORT ON THE KERATOSA. 49 



have seen these elements so strikingly minute. This peculiarity renders the sections of 

 them readily distinguishahle under the microscope from those of any other Porifera, and my 

 first idea when examining the preparations of Psammopemma porosum was that this form 

 might represent an offspring of the family above mentioned. Yet an attentive examination 

 showed that the species has even more right to be referred to the Keratosa than Psammo- 

 pemma densiim ; for while in this latter form the horny substance enveloping the foreign 

 enclosures has been found in the form of an extremely thin layer, so that its presence 

 beyond doubt could only be proved in most instances after treatment with hydro-fluoric 

 acid, here in Psammopemma porosum it proved to he far more fully developed, and 

 occasionally with very conspicuous outgrowths. 



On p. 17 1 have shown that on the whole, as the canal-system reaches a higher 

 degree of development, the size of the flagellated chambers gradually diminishes. Of 

 course the flagellated chambers of Psammoclema vosmaeri are not larger than those, for 

 instance, of Euspongia officinalis, and yet they are devoid of special cameral canaliculi, 

 but every rule has its exceptions. On the ivhole, the above dependence is still to be 

 observed, and it is not without interest that as flagellated chambers become smaller 

 and smaller the flagellated cells in their turn grow gradually more and more minute. 

 The flagellated cells of Apli/silla sulphurea are larger than those of Spongelia elegans, 

 which again are larger than the flagellated cells of Cacosp>ongia scalar is or Aplysina 

 aerophoba. But, as in the preceding case, exceptions are not wanting here also ; lanthella 

 flabelliformis does not deviate in the structure of its soft parts from the type 

 characterising Aplysilla sulphurea, and yet possesses flagellated cells of far smaller 

 dimensions than those of the form just mentioned. Again in Euplectella aspergillum, 

 in spite of its large, radial tube-like flagellated chambers, these cells are very minute. 

 Psammopemma porosum presents, in this respect, the most interesting exception. As 

 stated before, its flagellated cells are very small ; while for instance in Cacospongia 

 scalaris the diameter of their transverse section is 0"0025 mm., on an average in 

 Psammopemma porosum it does not exceed O'OOl mm. ; and yet its flagellated chambers, 

 although smaller than those of Psammopemma densum, are rather larger than those of 

 Cacospongia scalaris or Euspongia officinalis. The explanation of this curious deviation 

 must be of course left to later investigations, but there can be scarcely any doubt that, 

 if not even of a subgeneric importance, at any rate it necessitates the establishment of a 

 new species, although on the whole the internal organisation of Psammop>emma porosum 

 agrees closely with that of Psammopemma densum. As a probably accidental 

 peculiarity I can notify the presence in the parenchyma of numerous round bodies, with 

 an average diameter of 0'012 mm., which at first sight recall vividly the spermospores of 

 Calcarea or Aplysinidae (p. 72). Under high microscopic j)ower these bodies proved 

 however to be devoid of any covering cell, and though their contents are apparently 

 cellular elements and their size approximately the same as that of the spermospores of 



(ZOOL. CHALL. EXP. — PART XXXL — 1884.) Hh 7 



