The skeleton of Placospongia melobesioides and carinata (PI. II, fig. 2) consists of a 

 very hard central axis, composed of sterrospirae ] ) ; bundies of tylostyles radiate from the axis 

 towards the periphery, without projecting beyond the surface. The chief mass of the cortical 

 skeleton consists, like the axis, of densely packed sterrospirae. In the parenchyma a large 

 quantity of these spicules are found in various stages of development. It seems, therefore, 

 that they are formed in the parenchyma in order to be transported towards the axis and the 

 cortex, vvhere the adult ones are deposited. Lining the distal part of the cortex, as well as 

 the oreater part of the canals, very minute spicula are found which are different in the two 

 species. Entering the cortex at places where there are grooves, the bundies of tylostyles split up 

 into thinner bundies (PI. II, fig. 6). Tangential sections through the proximal portion of the 

 cortex show us the distribution of such bundies (PI. II, fig. /\.ó). In the distal portion of the 

 grooves the tylostyles, much smaller here, project somewhat beyond the surface, being arranged 

 in the shape of reversed cones. 



The tissue of the cortex is always distinctly separated from that of the parenchyma. 

 The greater part is so crowded with sterrospirae, that hardly anything else is visible ; only in 

 the most proximal part circular fibres may be seen. In the grooves a rather complicated system 

 of contractile fibres is met with, especially in the proximal three quarters. Here we observe 

 numerous fibres, arranged concentrically, and in addition to these some groups of fibres in other 

 directions : longitudinally, obliquely, radially. We have given illustrations of these contrivances 

 in figs. 1 and 3 on PI. II. The same sort of fibrous bundies accompany the radiating bundies 

 of tylostyles (PI. III). 



So far P. melobesioides and carinata agree. With regard to the spiculation there are 

 striking clifferences. In P. melobesioides the dermal spiculum is a spherule, generally smooth, 

 sometimes slightly roughened or otherwise modified. In P. carinata this spiculum is a microspira 

 (reduced sigmaspire or spinispire?). In the latter species we find numerous spinispirae with short 

 axis and long spines (the spirasters, metasters or plesiasters of Sollas), whereas these spicula 

 are entirely absent in P. melobesioides. Since these facts are absolutely constant and never 

 show any transition, we are obliged, according to the views of Hugo de Vries, to distinguish 

 two species. 



III. THE STERROSPIRAE OF PLACOSPONGIA. 



Grav, the father of the genus Placospongia, compared the "siliceous globules ' of 

 his new genus with the so-called "ovaria" (sterrasters) of Geodia ; this view was generally 

 accepted, till Keller wrote ( 1 89 1 p. 298): "Analysirt man die Kieselkugeln naher, so ergiebt 

 sich sofort, dass dieselben aus Spirastern hervorgegangen sind. Man findet im Schwamm- 

 gewebe zwischen Kugeln, Spirastern und bedornten Staben alle möglichen Zwischenformen." 



1) Cf. p. 7. 



