202 PORIFERA. II. 



most of the papilla; liave a wide opening in the point. While in the former specimen the papilla; are 

 conical, they form in the latter a cylindric limb round the aperture. These papillae, no doubt, are 

 oscula. Besides these papillae, however, some other papillar formations are found; they are most 

 frequently broader, but lower than the oscular papillae, and consist of a low ridge surrounding a 

 shallow groove. In the membrane coating the groove pores are found more or less close-set; outside 

 these formations I have found no pores. Now the question is whether pores and oscula are thus 

 restricted, each to a set of papilla; of their own, or all the papilla; have a pore membrane, which is 

 torn off in the papillae showing an opening; in the latter case the low pore papillae would then be 

 less contracted stages of the higher or quite closed papillae. Then again the question would be 

 whether the papillae acted as incurrent or excurreut openings, or in both ways. It is, however, most 

 probable that there are two kinds of papilla;, one kind with oscula, the other with pores; the con- 

 struction indicates especially that the low pore papillae cannot be closed as the closely shut oscular 

 papillae. 



The skeleton. The dermal skeleton consists of bundles of dermal spicules projecting from the 

 skeleton below; they are oftenest erect, sometimes a little recumbent. They may be more or less 

 close-standing; where they are farther from each other, comparatively large parts of the membrane 

 are thus found having no dermal spicules, and only provided with microscleres. In the mentioned 

 papilla; which are formed by the skin, the skeleton is altered. The spicules are lying horizontally 

 in the wall of the papilla, and consequently they do not rise over the surface; they are parallel to the 

 longitudinal axis of the papilla, and are gathered into bands; the general arrangement of the dermal 

 skeleton in projecting bundles passes at the papillae by degrees into the band-shaped arrangement; 

 the spicules are numerous and very close-lying, towards the point the bands converge; the closed 

 oscular papilla is somewhat spirally twisted. In the pore papillae the same construction is found, the 

 only difference being that the band-shaped arrangement is less distinct, but the spicules are lying 

 close together in the low ridge; besides some fibres, at all events in the larger pore papillae, pass 

 under the membrane of the groove, and send off spicula-bundles supporting the membrane and pro- 

 jecting a little. The main skeleton consists of a rather irregular network. Primary fibres are found, 

 running in the longitudinal direction or towards the surface, but they are somewhat irregular and 

 loose ; transverse fibres are not formed, but transverse spicules are found, most frequently placed singly 

 and very irregularly. The longitudinal fibres are polyspicuiar, and have generally three to six spicules 

 alongside. Spougin is found where the spicules are united, but it is present only to a slight amount, 

 and is white and clear, so that it is only to be seen with difficulty. 



Spicula: a. Megasclera. i. The skeletal spicules are styli; they have a more or less marked, 

 most frequently rather slight, curve, generally situated near the head-end. The point may be a little 

 varying, but may best be described as middle long, it is oftenest distinctly bounded. Their length is 

 °'53"°7 I 5 mm i an d the thickness is 0-0128— o-02i mni . Some developmental forms were seen, the youngest 

 ones are long pointed. 2. The dermal spicules are tylota; they are straight, and their ends are 

 most frequently rather slightly swollen. Their length is 0-31— 0'45 m "\ and the thickness 0-007— o-oi2 mm . 

 The shaft is a little thicker in one end than in the other, and the swelling of this thicker end is most 

 frequently so slight, that the spicule might be called a tylostrongyle; the swelling of the thinner end, 



