Additional remarks. A very striking feature of Placospongia is that it is ver)- hard 

 and stony, owing to the fact that it possesses a thick siliceous axis, and a similar cortex, both 

 composed of closely packed sterrospirae. 



II. GENERAL ANATOMY. (PI. II— III). 



A transverse section through a branch of P. mclobesioides or cariuata (PI. V, fig. S) 

 shows a sharply marked cortex at the periphery and a likewise sharply marked axis in the 

 centre of the parenchyma. This is especially distinct in red specimens; axis and cortex are 

 red-brown, whereas the parenchyma is of a yellowish tint. It will, however, easily be seen that 

 the red cortex is not continuous ; it is interrupted in several places by almost white bands, 

 about i mm. broad. These interruptions of the red cortex are nothing but the longitudinal 

 grooves, cut transversely, which, although belonging to the cortex are destitute of sterrospirae. 

 One may count ten or more such places on a transverse section, dividing the whole cortex 

 into as many portions of unequal size. Sollas (1888 PI. XL, fig. 7) and Keller ( 1 89 1 

 PI. XVIII, fig. 30) give illustrations of transverse sections, apparently of young branches, for 

 in both cases the cortex is only divided into four portions. The general outline of transverse sections 

 is, however, as a rule, not square but polygonal. The outline of the axis is not a circle but 

 a lobed figure. It is in some sections continuous with the cortex, which represents a beginning of 

 ramification. Erom the axis we can, still with the unassisted eye, easily see white bundies 



(of tylostyles), radiating towards the periphery. 



A longitudinal section (PI. V, fig. 9) shows, mutatis mutandis, the same. Only we see 

 here the red cortex less frequently interrupted, and, if so, the cortical grooves are generally 

 cut longitudinally and consequently of much greater dimensions. 



As stated before, the incurrent apertures are situated in the grooves. These are in most 

 specimens exceedingly narrow, owing to the contraction of the cortical fibres. In one specimen, 

 however, they were but little contracted and we have, therefore, chosen this specimen (P. mclobe- 

 sioides 1033) for the greater part of our drawings. The numerous stomata are the entrances 

 of narrow canals, some of which unite with neighbouring canals, and form somewhat wider 

 ones, but they debouch all into two or three longitudinal canals (PI. II, fig. 6; PI. III). From 

 these canals start others in more or less radial direction and enter the parenchyma. They are 

 in communication with each other by circular canals, which are sometimes very long indeed. The 

 system of incurrent canals in one groove may communicate in the parenchyma with that of another 

 groove (PI. II, fig. 2 ; PI. III). From the moment the canals enter the parenchyma, they give off 

 much narrower branches, which ramify and terminate by means of prosodi into ellipsoid-shaped 

 mastichorions '). From here the water is moved through aphodi into narrow excurrent canals, 



1) We propose to call a region, bearing choanocytes, as e. g. in L e u cos o Ie n ia: mastichore ; the localiscd small regions, 

 known as ciliated or flagellated chambers, we wish to call mastichorions, in order to have a shorter expression which can be used in 

 every language. The words are derived from /.(ia-T/s, flagellum and x"P x i dimin. %oi(iov , region. 



