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



shells. Having dissolved the calcareous matter by cautious application of dilute hydro- 

 chloric acid, I was able to examine in a rather satisfactory manner the delicate remains, 

 consisting of a scarce, clear maltha, and of the branched canals traversing the latter. 

 The canals have often a distinct membrcma propria, the wall of which is supported by small 

 xenophya (fig. 70). Numerous rather large flagellated chambers, of an ovate or oblong 

 form, were visible between the smaller branches of the canal-system, and partly connected 

 with them ; here and there, too, the small inhalent canals could be recognised coming 

 from the small dermal pores. The choanocytes were exceedingly small, on an average 

 O'OOl mm. in diameter ; the same was observed by Polejaeff in Psammopemma porosum. 

 The best preparation of the canal-system was obtained by vertical sections through the 

 discoidal Psammina plakina (PI. VII. figs. lC, ID). In this remarkable preparation 

 were also found single eggs, some in segmentation (figs. lC, ID, e). Their structure and 

 disposition are similar to those in Plakina monolopha. 



The excurrent part of the canal-system exhibits in the deep-sea Psamminida? three 

 different types. The discoidal Psammina possesses a girdle of oscula, or of large 

 exhalent openings (usually between ten and twenty) on the peripheral margin of the 

 medal-shaped body (PL VII. fig. 2B). The tuberose Holopsamma bears either a single 

 osculum on the top of each prominent lobe (fig. 6B, o), or a series of oscula (or several 

 series) on the projecting crests of the massive body, between the conical depressions 

 which bear the dermal pores (fig. 7 A). The true Psammopemma has no distinct oscula 

 at all (figs. 4, 5). 



Symbiontes. — The majority of the deep-sea Psamminidse are not associated with a 

 symbiotic Spongoxenia. Two species only of Psammina exhibit this symbiosis, viz., 

 Psammina globigerina (PL VII. fig. 2C) and Psammina nummulina (fig. 3). Between 

 the two parallel hard dermal plates of these discoidal sponges (which in the former are 

 composed of Globigerina ooze, in the latter of Eadiolarian ooze) is placed a soft medullar 

 mass, with the canal-system of the sponge, and within this is expanded a network of 

 anastomosing chitinous tubes, filled with dark brown cells (figs. 2C, 3). This is probably 

 the hydrorhiza of a symbiotic Hydroid (Stylactis ?) ; its hydranths and gonophores, 

 however, could not be seen. 



Genus 4. Psammina, 1 n. gen. 



Definition. — Psamminidge with a discoidal body, forming a thin and flat crust or 

 plate, the margin of which is provided with a series of oscula. The canal-system is 

 expanded horizontally in a soft medullar mass, which is enclosed between two hard 

 cortical plates (upper and lower plate), both full of xenophya. 



The genus Psammina, represented in the Challenger collection by three new and 



1 Psammina = Sandy, -J. a «,«<>«. 



