204 CEYLON PEARL OYSTER REPORT. 



Darwinella simplex, TOPSENT, 1892 (84, vide also 62) -Plate XV., figs. 1, 2. 



The single specimen forms a thin crust extending over a considerable area on the 

 surface of a branching tube which has apparently belonged to some annelid worm. 

 The surface of the sponge is glabrous and covered with sharp-pointed conuli, about 

 1 to 1*5 millims. in height and some 2 to 3 millims. distant from one another. The 

 maximum thickness of the crust is only about 2 millims. The dermal membrane is 

 minutely reticulate and lifted up in a tent-like manner on the ends of the vertical 

 skeleton fibres to form the surface conuli. The colour (in spirit) is dark purple,* and 

 the texture very soft and tender. The vents are inconspicuous, only one small one 

 having been detected. The inhalant pores are abundantly grouped in pore-sieves, 

 which occupy the oval or rounded meshes of the dermal reticulation. 



The skeleton consists, in the first place, of sparingly and irregularly branched, 

 pithed horny fibres (Plate XV., figs. 1,2) of the usual Darwinella type, which rise 

 more or less vertically from the base of the sponge and terminate in rounded apices 

 in the surface conuli. These fibres have a diameter of about 0'165 millim. near the 

 base, diminishing to about half as much in the conuli. The base of the fibre is 

 expanded into a thin plate of spongin, doubtless attached to the substratum ; the 

 fibres themselves do not appear to form any anastomoses. The wall of the fibre 

 is only about O'Ol millim. thick, and the interior is generally occupied by a much- 

 branched filamentous fungus (?), composed of rows of short cells, which more or less 

 completely replaces the pith in the older parts of the fibre (Plate XV., fig 2). In the 

 younger parts of the fibre (Plate XV., fig. 1) the pith exhibits the characteristic 

 thimble-shaped layers described and figured by LENDENFELD in D. aurea. 



In the second place we have horny spicules of the usual Darwinella type, but 

 rather sparingly developed and, so far as I have been able to ascertain, all of the 

 triradiate form. They are irregularly scattered through the soft tissues, and I have 

 seen no union between them. The rays are long, slender and tapering ; say about 

 0'5 millim. long by about 0'025 millim. thick near the base. 



The canal-system and general anatomy agree very closely with the description and 

 figures given by LENDENFELD (66) for D. aurea. 



The Ceylon specimen agrees very well with TOPSENT'S descriptions of the species, 

 but the horny spicules (in the single specimen available) do not appear to attain so 

 large a size. The species has hitherto been recorded only from the Mediterranean 

 and the Azores. 



B.N. 302 (Ceylon seas). 



Megalopastas, n. gen. 

 Aplysillidae with an entirely reticulate skeleton and without spongiu spicules. 



As I have already pointed out, LENDENFELD (66) includes in his 'genus Dendrilla 

 both species (e.g., D. rosea) without and species (e.g., D. elegans) with a reticulate 



* Vide footnote under lotroehofo p 



