38 THE VOYAGE OF H.M.S. CHALLENGER. 



The exhalent oscula, ten to fifteen in number, are larger, and form a corona on the 

 elevated peripheral margin of the disc (as shown in PI. VII. fig. 2B). The diameter of 

 the disc is 12 to 15 mm., the thickness 1'12 to 1'18 mm. 



The two hard parallel dermal plates of the disc are easily detached from each other, 

 and then is seen between them a softer medullar plate, composed of the branched canal- 

 system of the sponge, and of the network of the symbiotic Spongoxenia (PI. VII. fig. 3). 

 The structure of the canal-system is difficult to make out, but seems to be simUar to that 

 of Psammina globigerina (fig. 2D). The inhalent pores on the upper face of the disc 

 open into small canals, and the main tubes of the canal-system open on the elevated 

 margin of the disc by exhalent oscula. 



The symbiotic Spongoxenia (fig. 3, h) (probably the reticular hydrorhka of Stylactis or 

 an allied tubularian Hydroid) forms an elegant network with polygonal meshes, expanded 

 horizontally in the equatorial plane of the disc between the branches of the canal - 

 sj^stem. The anastomosing chitinous tubes of the network are filled by a dark green- 

 brown cellular detritus, sharply defined from the whitish tissue of the sponge. 



Genus 5. Holof)samma, 1 Carter (1885). 



Definition. — Psamminidse with a massive tuberose or lumpy body, which bears 

 groups of distinct oscula either on prominent ridges or on the top of projecting lobes. 



The genus Holopsamma was founded in 1885 by Carter with the following defini- 

 tion : — " Arenaceous sponges without fibres, whose composition consists of foreign 

 microscopic objects (sand, fragments of sponge-spicules, &c.) diffused in the flakes of 

 the parenchymatous sarcode, traversed by the canals of the excretory system." 2 

 Carter points out that " there is absolutely no fibre, but the foreign material is 

 diffused, and so far held together bv beins imbedded in the delicate flakes of the 

 parenchymatous sarcode" (i.e., the maltha, or the ground-mass of the mesoderm). 

 Carter describes five different species of Holopsamma ; the three first of these are 

 characterised by a massive lumpy or tuberose body, in which numerous distinct oscula 

 are visible, usually placed on the most projecting parts, either on the margin of crests 

 or the top of lobes. These three typical species of Holopsamma are Holopsamma 

 crassa, Holopsamma laevis, and Holopsamma laminsefavosa. To these are closely 

 allied two new deep-sea species obtained by the Challenger, and described in the 

 following pages (Holopsamma cretaceum and Holopsamma argillaceum). The two 

 remaining species of Carter might be better placed in the genus Psammopemma of 

 Marshall. 



1 Holopsamma = Whole sand, oAo?, day-px. 

 - Loc. cit., p. 211. 



