PORIFERA. II. 



167 



ca. o-2 mm up to ca. i mm . These openings lead into canals passing horizontally inward and losing them- 

 selves in a net of smaller canals. These larger openings are snrely oscula; but as it is seen, all the 

 openings pass evenly into each other with regard to size, and so it is often not to be decided whether 

 an opening is incurrent or excnrrent. 



The skeleton. The dermal skeleton. As mentioned above, the dermal membrane rests on the 

 skeleton below, and the fibres of the skeleton project through it, by which means the surface of the 

 sponge is made shaggy. Otherwise the dermal skeleton is formed by dermal spicules in the common 

 way; partly these spicules form bundles projecting from the main skeleton, often in connection with 

 the projecting ends of the fibres, but mostly they are lying horizontally in the dermal membrane, 

 partly scattered singly, partly in bundles, and in the pore areas they are often seen branching in 

 between the pores; upon the whole, however, the dermal skeleton must be said to be little developed. 

 The main skeleton is a somewhat irregular, mostly polyspicular network. Longitudinal fibres are found, 

 running in the longitudinal direction of the branches, and by degrees bending out and piercing the 

 dermal membrane, or giving off branches that bend out in this way; these primary fibres are all poly- 

 spicular and have an average thickness of 0-07— cri2 m,n , and their course is rather regular. By far the 

 greatest part of the transverse fibres is only represented by single spicules, only here and there a few 

 are seen alongside each other; these spicules are placed quite irregularly, and coherent fibres are not 

 formed. In a longitudinal section the skeleton so far conveys the impression of some regularity, as 

 the longitudinal fibres are running rather parallelly, and the same is the case in a transverse section, 

 the ends of the fibres bending towards the surface being here seen, especially in the circumference 

 of the section, as rather regularly radiating radii. In the skeleton a rather copious, slightly yellowish 

 mass of spongin is found, wrapping the fibres completely. Down towards the base, and especially in 

 the stalk of the individuals provided with a stalk, the spongin is very abundant and forms a thick 

 sheath round the fibres; consequently the sponge is here firm and hard. 



Spicula: a. Megasclera. 1. The skeletal spicules are smooth styli; they are slightly curved, 

 and the curve is most frequently found nearest to the upper end; they are evenly and middle long or 

 rather long pointed. No head-swelling is found, but the greatest thickness is generally situated at 

 the very head-end, and here the axial canal shows a small, but distinct widening, not situated just at 

 the end of the canal, but a little below it. Their length is somewhat varying, from 0-42 — o , 68 mni , and 

 the thickness from 0'0i6 — o - 025 ram . A few developmental forms were seen; they show especially dis- 

 tinctly that the head-end is a little thickened, and in the finest ones a swelling is found, often placed 

 a little below the end. 2. The dermal spicules are strougyla with transitions to tylota; a discern- 

 ible swelling of the ends are almost always found, but it is always slight, so that the needles may 

 perhaps be best characterized as subtylota. They are straight, more rarely slightly curved. As is 

 generally the case, the two ends are not quite equal, one being a little thicker than the other, and a 

 few, quite fine developmental forms showed the thinner end quite pointed. They are somewhat varying 

 in size, the length from o - 22— o^o" 1 ™ 1 , and the thickness from 0-0035 — o-oo7 mm . The variation is not found 

 in its whole extent in one and the same individual, thus there are individuals in which the dermal 

 spicules do not exceed o - 33 mn ', and others in which they do not go below o^S" 1 ™ b. Microsclera; these 

 are of three forms, chelae arcuatae and sigmata of two sizes. 1. The chelae arcuatae have chiefly 



