THE PORIFERA OF THE SIBOGA-EXPEDITION 



i. 



THE GENUS PLACOSPONGIA 



BY 



G. C. J. VOSMAER and J. H. VERNHOUT 



\Y i t h f i v e p 1 a t e s 



Gray described (1867 fi p. 127 — 129) a very remarkable sponge for which he established 

 the new genus Placospongia. "The bodies have much the appearance of the underground 

 rhizome of a plant, with a number of scars whence leaves or flovvering branches have separated ; 

 but when more closely examined, it will be found that what appears to be a scar is a separate 

 plate. And when so examined they have so much the appearance of a very large kind of 

 Nullipore or Melobesia that, when I first observed them, I believed ihat they were probably 

 corals covered with large plates of a Melobesia . . . ." Gray called, therefore, the species melo- 

 besioides. The author further states that "the sponge (is) hard, angular, stony, angularly 

 branched", that it is composed of a solid axis of siliceous globules, around which is a layer 

 of "sarcode", which is then "covered with variously shaped hard plates of similar tubercular 

 siliceous globules". Bundies of pin-shaped spicules run from the axis towards the periphery. 



This description, accompanied by a woodcut, sufficiently enables us to recognize the 

 sponge at once. 



Though specimens of Placospongia have been studied by many spongiologists, our knowledge 

 of the anatomy of this genus is still very insatisfactory. This may be due to the fact that 

 only a few specimens, perhaps not even well preserved, were at the disposal of the investigators. 

 The Siboga, however, has brought home a fine collection of numerous specimens. 



Externally, the two species we found in this collection, cannot be distinguished from 



SIBOGA-EXPEDITIE VI a. 



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