46 THE VOYAGE OF H.M.S. CHALLENGEK. 



Psammopemma densiim, Marshall (PI. III. figs. 3, 4). 



Psammopemma densum, Marsliall, Zeitschr. f. wiss. Zool., Bd. xxxv. p. 116, 1880. 



When characterising this species, Dr. Marshall did not feel quite certain whether he 

 was really describing a sponge and not a worm-tube or something of that kind. Of 

 course, I have scarcely the right to express an opinion on this delicate question, still I 

 believe it to be a sponge; at any rate I can state wdth the greatest certainty that such sponges, 

 i.e., sponges ^dthout any differentiated skeletal fibres but still secreting horny substance, 

 do really exist. Sanctioning thus the establishment of the genus Psaimnojxmma, I 

 adopt also Dr. Marshall's species Psammopemma densum, since its specific designation 

 is very characteristic of two specimens in the Challenger Collection, and especially of that 

 from Port Jackson. 



This last specimen is represented in PL III. fig. 3, and it wdl be obvious from this 

 drawing that the original is probably not the whole animal but only a fragment of it. 

 Now even if this fragment give no jjrecise idea as to the shape of the whole animal, it 

 must still be assumed that this latter was of a plate-Hke compressed form, supposing 

 the fragment in question was not merely an outgrowth ; while the external shape of 

 the second Challenger specimen, represented also only by a fragment, must have 

 been either crust-like or massive, the plane of the fracture being parallel to its outer 

 surface. 



So far as this latter is concerned, it is in both specimens uneven and throughout its 

 whole extent rough, owing to prominent sand-grains, often 1 mm. in diameter. In the 

 Australian specimen, at one point on its surface, I found a shallow dej)ression, which I am 

 inclined to regard as the osculum ; but I refrained from trying to prove it by immediate 

 dissection lest I should destroy the specimen in vain, the sponge presenting such a com- 

 pact aggregation of sand-grains that only very thick sections could have been obtained 

 from it by the microtome. Besides, the question is of little consequence. 



In contrast to the specimens which Dr. Marshall had for examination, both the Chal- 

 lenger specimens proved to be free from any parasitic inhabitants, as well as from any 

 skeletal fibres, either foreign or produced by the sponge itself. Nor can I say that the 

 foreign enclosures are held together by protoplasm as stated in Marshall's definition of the 

 genus (loo. cit, p. 113); each of them, though surrounded by a thin horny envelope, having 

 been found lying separately in the parench^ona. That the envelope just mentioned is of 

 a horny nature I judge from the fact that it does not differ from the envelopes to be 

 found around foreign enclosures in other Spongelidse, lying free in the " sarcode," these 

 latter in their turn not differing from the envelope of true skeletal fibres overcharged 

 with foreign bodies. Contrarily to Marshall {loc. cit., p. 114), I find this envelope to be 



