EEPOET ON THE MONAXONIDA. Ixi 



Homorrliaphidse (Chalininag), in the Heterorrbaphiclse {Gelliodes, Toxochalina), in the 

 Desmacidonidse {Esperella, &c.), and in the Axinellidse {Axinella fhrosa) , hand in hand 

 with a corresponding reduction in the siliceous element. We thus know four distinct 

 paths along which the Keratosa may have developed, and the group is thus shown to 

 be probably of polyphyletic origin, and, consequently, unnatural.' 



This fact probably accounts for the singular difficulty which Polejaeff found in 

 classifying the group, and appears to us to be a much more probable explanation of this 

 difficulty than the assumption, for we can caU it little else, that " the whole group is 

 nothing more than a simile family."'^ This method of cutting the Gordian knot, simple 

 as it is, is hardly satisfactory. 



Having come to the conclusion that the Monaxonida do not constitute a very natural 

 order, although the two suborders therein included are probably natural enough, we 

 ought perhaps to attempt some justification of our conduct in retaining the name at all. 

 The real fault lay in the original distribution of the Challenger collections, and this could 

 not be avoided, for it is only since this distribution was made, and since two of the 

 Eeports on Sponges have been published, while others have been far advanced, that the 

 great mass of facts necessitating the recent modifications in classification has been 

 brought to light. At one time it was believed that the Monaxonida were a natural 

 group, and by the time that the error was discovered the work was so far advanced that 

 it was impossible to efi'ect a redistribution. Hence we were left with two suborders, the 

 Halichondrina and the Clavulina, and for these we have been forced to retain the name 



1 Marshall's Plioriospongise, which have given rise to so much discussion, are to be similarly explained as having 

 originated polyphyletically from the Halichondrina. Marshall gives the following diagnosis of the genus : — 

 " Kieselschwiirame mit sehlanken, einfachen Nadeln mit einer Spitze, Stecknadeln und Doppelhaken durchziehen imd 

 umspinnen Sandmassen, sie zu Klumpen vereinigend; das Ganze ist mit einer ahziehbaren Haut bedeckt" {Zdtschr.f. 

 wiss. ZooL, Bd. xxxv. pp. 122-126). The view proposed by him that they are siliceous sponges which penetrate and unite 

 together masses of sand appears to us hardly to bear investigation ; the fact that the whole mass is enclosed in a definite, 

 pore-bearing, dermal membrane, as Marshall himself describes, is opposed to this idea. Von Lendenfeld {Proc. Linn. Soc. 

 N.S.W., vol. X. p. 81) advocates a very different hypothesis; he says " I do not hesitate to consider the Phoriospongiai as 

 belonging to the horny sponge as well as those Porifera which, like Dysidea, possess an arenaceous skeleton but no flesh- 



spicules I consider the Phoriospongia3 not as boring sponges living in sand ; but as Ceraospongiaj belonging to the 



groupwith arenaceous irregular fibres,"and again (p. 84) — "I believe that the flesh-spicules in thePhoriospongiajand horny 

 sponges on the one hand, and those of the silicifilired sponges on the other have been produced independently of each 

 other." It seems to us that von Lendenfeld also has here placed a wrong interpretation upon the facts before him. 

 It is quite unnecessary to assign such a polyphyletic origin to the microsclera ("flesh-spicules ") in question, and we regard 

 the Phoriospongise not as forming a separate genus at all, but as derived from several distinct genera of Monaxonida, 

 in which, probably owing to the influence of similar external conditions, the proper siliceous skeleton has been replaced 

 to a greater or less extent by sand and other foreign bodies. The fact that when proper megasclera occur in these 

 sand sponges they are small and slender, and to all appearance degraded forms, argues in favour of our view. 

 It is well known that sponges have a strong tendency to take in foreign bodies of all kinds with which to build up a 

 skeleton. In the horny sponges (e.g., Euspongia, Dysidea) this very frequently occurs, and also in the Heterorrhaphidffi 

 (Tedania commixta, nobis), the Desmacidonidje {Esperella parasitica, Carter, Esperella arenicola, nobis, lophon omnivorus, 

 nobis), the Suberitidre {Polymastia agglntinans, nobis), and the Spirastrellidae {Spirastrella solida, nobis). In some cases 

 these sand sponges have sufiicient spicules remaining to enable us to say from what genus they have been derived, 

 while in other cases this is no longer possible. 



2 Zool. Chall. Exp., part sxxi.. Report on the Keratosa, p. 81. 



