84 



PORIFERA. I. 



other side of the sponge that has no oscula, is the pore-side. The pores are here very close-set, they 

 are more or less circular, and are lying in groups in the fields formed between the bundles of spicules 

 that are spread in a penicillate manner. Their size is between 0-03 — o-og"™. In this species pores and 

 oscula are thus execlusively localized each to their own side, and in connection with this fact the two 

 sides show a somewhat different structure of the skeleton. To judge by the specimens in hand it 

 seems to be the more or less convex side that is provided with oscula; it was to be expected that 

 the convex side was the one turned downward or outward, and then oscula would be found on the 

 outside; but whether this be the fact cannot be decided, until a whole specimen is obtained for 

 examination. 



The skeleton consists of an irregular network of spicules crossing each other in every direction, 

 and between them loose fibres are found. Spongin is not observed. Towards its two surfaces the 

 skeleton, however, has a particular structure, short fibres being found here running more or less ver- 

 tically on the surface; in the outer end of these fibres the spicules spread in a penicillate or funnel- 

 shaped way, and pierce the dermal membrane. When these fibres are not vertical on the surface, they 

 generally bend a little arcuately outward towards the outer edge of the sponge. As said before, there 

 is some difference between the two sides; on the oscular side these fibres which support the dermal 

 membrane as columns, are shorter and thinner, accordingly they generally consist of fewer spicules, 

 and they are closer together than on the pore-side. On this side where the columns are somewhat 

 longer and farther from each other, we therefore see in a transverse section just below the skin a 

 great many subdermal cavities separated by the columns; in reality they all form presumably one 

 large subdermal cavity. Also on the oscular side the skin is supported by the columns, so that we 

 also here find a cavity, but a much smaller one, below the skin (PI. XV, figs. 8 — 9). The skin on the 

 pore side comes off very easily, and when it is off tlie numerous incurrent canals are seen; they are 

 of the same width as the excurrent canals, and in pieces where the dermal membrane is wanting, we 

 may easily be led to suppose that oscula are found on both sides. The incurrent canals and the ex- 

 current ones go, each from their side of the sponge, through it almost to the opposite side; they are 

 often somewhat bent, and also often branched; they are of the same width, in the principal canals it 

 is ca. I'"". In the irregular middle parts of the siDonge the canals have also a more irregular course. 



Spicitla: a. Megasclera. These are long and slender subtylostj'li or tylostyli. Sometimes they 

 are straight, but more frequently slightly curved; the curve may be somewhat varying, and is not 

 rarely found nearest to the head end. The head may be marked off to a different degree from a 

 scarcely perceptible swelling to a fully developed, round head, so that the needle becomes a marked 

 tylostylus; sometimes the swelling is found a little before the end. The other end is evenly produced 

 to a long, rather fine point. The needle is thickest in the middle, also tapering somewhat towards 

 the head end. The size of the needles is very much var\-ing; the most common length seems to lie 

 between o-65™"> and 077'"'^, but it may increase to 0-89""" and go quite down to 0-203™"', which is the 

 smallest size I have measured. Like the length the thickness is also very much varying; the largest 

 spicules have in the middle a thickness of O'Oij"™, and the smallest go down to 0-004""". Upon the 

 whole the thickness is in proportion to the length, but among the longer forms rather tliin ones may 

 also be found. Quite fine spicules, developmental forms, are found of different lengths, but in small 



