both selenasters and pseudosterrasters, are only elongated forms of sterrospirae, not spicula 

 sui generis x ). 



Another peculiarity of the sterrospires of Placospongia melobesioides and P. carinata is 

 that they are often coloured. 



It is indeed astonishing that nobody has ever mentioned this fact. Not only is it - - as 

 far as we know — the only example of coloured spicula in Sponges, a fact very common in 

 calcareous spicula of some Alcyonaria — but the colour of the Placospongia itself is due only 

 to the colour of the sterrospirae. However, not every sterrospira is coloured, and the intensity 

 of stain is not ahvays the same, though there is not much clifference in this respect. Roughly 

 speaking we can say that there are sterrospirae, colourless like other spicula, and others 

 which, seen under the microscope, have about the colour of a living red blood corpuscule. 

 Both kinds are mixed, and the total colour of the sponge depends on the percentage of 

 coloured sterrospirae. In "red" sponges this percentage is about 100%, in "white" specimens 

 almost or quite o.'„ (163 b). 



In a glass tube filled with dry, isolated "), coloured sterrospirae, these look in reflected 

 light, like very fine sand of a flesh colour. YVhen moistened they turn red. As the red colour 

 of the sponge is only due to the sterrospirae, dried specimens will always look white or rlesh- 

 coloured; the red specimens will, after moistening, turn red, the others remain pale. Neither 

 alcohol, nor potash, nor muriatic acid destroys the colour. The turning red of dry specimens 

 by moistening is a physical, not a chemical phenomenon. 



IV. THE SPECIES OF PLACOSPONGIA. 



Five species are described as belonging to Placospongia • a sixth is called Physcaphora 

 decor ticans. These six species can, however, be reduced to three, viz. Placospongia carinata, 

 P. melobesioides and P. decorficans. 



1. Placospongia carinata (Bwk.) Rdl. PI. I, fig. 1 — 4; PI. II, fig. 5; PI. IV, fig. 9 — 13; 



PI. V, figs. 1, 5, 7—9, 11. 



Running numbers : 163 a, 163 b, 311 b, 311 c, 311 e, 311 f, 577, IOO4, 1458, 1500, 

 1848, 1850, 1852, 1854, 1855, 1856, 1857. 



Synonymy and literature. 



Geoclia carinata — Placospongia carinata. 



1858 (*) Bowerbank pp. 308, 314. PI. XXV, fig. 19; PI. XXVI, fig. 10. 



1861 Ehrenberg (tabul. view opposite p. 452). 



1) Cf. about the identity of Physcaphora decorticans and Placospongia graeffei infra p. 15. 



2) In spicula of a red specimen treated with muriatic acid, washed out and half dried, it is easy almost to isolate the sterro- 

 spirae. They are the fïrst to sink to the bottom of the vessel: if the water is then drained off and almost evaporated, the shining 

 white tylostyles can be most easily separated from the underlying reddish sterrospirae. 



SIBOGA-EXPED1TIE VI ff. 2 



