PORIFERA. II. 



213 



these, however, papillae occur in all degrees of contraction, down to quite small, knob-shaped projections; 

 the somewhat contracted papillae show only the net of meshes and pores at the top, and the entirely 

 contracted, low knobs have neither the net of meshes nor pores. It is, of course, impossible to tell 

 whether these different stages of the papillae represent stages of expansion and contraction, or they 

 are stages of development, when they have not been observed to open or close. There is, however, 

 every probability that the question is of contraction and expansion, as the quite low knobs show a 

 compact, more or less confuse accumulation of spicules, filling the knob entirely; then by degrees a 

 beginning of a net of meshes appears at the point, and the larger the papillae become, the more 

 expanded and regulated the net of meshes becomes. If the question was of a development of new 

 papillae, it was to be expected that the net of meshes was formed immediately, and that the papillae 

 only grew in size. Carter says: «Pores and vents respectively situated in the cribriform tubercles*, 

 and Vosmaer supposes that the tubercles have nothing to do with pores, but are exclusively oscula. 

 Now there can be no doubt that the papillae, to judge from their structure upon the whole, exclusively 

 carry pores, neither are pores found anywhere else in the skin. The real oscula have been overlooked 

 both by Carter and by Vosmaer, and they have not been mentioned by later authors either. These 

 oscula are formed as conical spouts, which are formed by the dermal membrane; they may be higher 

 or lower, sometimes they are very low, scarcely projecting, and then they are easily overlooked. The 

 dermal spicules are lying in the oscular wall parallelly to the longitudinal direction of the spout. 

 Oscula are only few in numbers in proportion to the pore papillae, thus on the largest specimen 

 ca. eight were counted. 



The skeleton. The dermal skeleton consists of very close-lying dermal spicules, all horizontal 

 and situated in several layers. They are not scattered, but form groups in which the spicules are 

 parallel to each other. On account of the dense spiculation the membrane is very solid. The skeletal 

 structure of the pore papillae and of the wall of the oscular spout has been mentioned above. To the 

 dermal skeleton belong further fibres of dermal spicules rising perpendicularly from the skeleton below 

 and supporting the membrane. The main skeleton is an irregular, to a great extent polyspicular, net- 

 work. Some fibres are found, running towards the surface; they are polyspicular, but may be of 

 rather varying thickness. When one succeeds in getting a section parallel to the direction of the fibres, 

 they are seen to run rather regularly, especially near the surface; in their outer part they consist of 

 dermal spicules, and pass out to and support the dermal membrane. Transverse fibres are not found, 

 but between the fibres a great many spicules are found, placed singly or a few together; they are 

 scattered quite irregularly between the fibres, so that the whole thing conveys a quite irregular 

 impression. A feature contributing to the irregularity of the skeleton is also the many, rather large 

 canals, between which the fibres must bend. Spongin is found in the nodes of the skeleton and in 

 the fibres, but it is white and clear. In the skeleton of the dermal membrane no spongin was 

 observed. 



Spicula: a. Megasclera. 1. The skeletal spicules are monactinal, most frequently styli, 

 but often with the point so much rounded as to become strongyla, in which case, however, they 

 preserve their monactinal character, one end being thinner than the other. They are slightly curved, 

 oftenest with the curve nearest the head-end, sometimes the curving is a little irregular. The point, 



