PORIFERA. II. 



J 35 



sponge, they are lengthened more inward, so as to become short fibres passing out and ending in the 

 dermal membrane. In the ridges of the surface that are more compressed and sharp-edged, the dermal 

 skeleton is often much suppressed, and the main skeleton may continue quite out and give rise to the 

 mentioned small spines. The main skeleton consists typically of triangular or, more properly, tetra- 

 hedral meshes, the sides of which have the length of a spicule and are formed of from one to five or six 

 spicules. It may, however, be somewhat differently constructed in different individuals. Thus more or less 

 marked fibres occur not rarely in larger or smaller numbers; they run especially in the direction towards 

 the surface. When these fibres occur in larger numbers, some quadrangular meshes may be formed here 

 and there, but a regular, quadrangular net of meshes is not formed. The longer fibres are generally 

 a little thicker than the others. In other individuals the skeleton is much less regular and is not 

 formed of so distinct triangular meshes, and then it is upon the whole of a more diffuse character. 

 In the membrane coating the canals only microscleres are found. Spongin is found in the nodes of 

 the skeleton, but it is only to be observed with much difficulty, as it is quite white and clear, and 

 only a minimal amount is present. 



Spietda: a. Megasclera. i. The skeletal spicules are acanthostyli, straight or slightly curved, 

 most frequently with the curve near the upper end; the point is middle long or rather short. The 

 spiuulation may be somewhat varying, but is generally rather scattered; the spines are not rarely 

 gathered a little more densely at the upper end; only a short part of the point shows no spines. The 

 styli vary rather much in size, not, however, in one individual, but in different individuals, whereas 

 the variation in one individual is most frequently only slight. The length varies altogether from 

 0-19 — o - 35 mm ; in the single individuals may be found for instance: 0-19— o - 22 mm , o - 2i — o-268 mm , 0-238 — 

 o-298 ram , and o'28— o - 35 mm . The thickness, which is only partly proportionate to the length, varies alto- 

 gether between crooS and o-oi5 mm . Finer, down to quite fine styli which are certainly developmental 

 forms, occur in most individuals scattered in the tissue. They have about the full length, and their 

 point is long and fine. Among the finest of them some were smooth, but otherwise traces of spinu- 

 lation occur early. In the fine forms the spines are small and especially low, so that these forms may 

 get an irregular, knotty appearance. In an individual with needles of the greatest lengths given 

 above, the fine developmental forms were measured, for instance, to 0'29 mm with a thickness of o-oo28 mm ; 

 the very finest one observed was o - ooio mm thick, and ca. o - i7 ram long; in this needle no trace of spinu- 

 lation was seen. From the finest stages all transitional forms are found to the fully developed needles. 

 2. The dermal spicules are tornota; they are straight, or at most quite slightly, irregularly curved. 

 They are cylindrical and most frequently slightly fusiform. The ends are short pointed; often a slight 

 constriction is found below the end; the ends are slightly spinulous, only in a few cases smooth or 

 almost smooth. It is to be remarked that the ends of the tornota are not always quite equal, but one 

 end is frequently a little thinner than the other. Their length varies in a similar way as that of the 

 styli, altogether it is between 017 and o - 26 mm , but in individual specimens it was measured, for instance, 

 to 0-17 — o-2o mm , 0-19 — o-22 mm , and 0-21 — o - 26 mm . The thickness is 0-0057 — o-oi mm . Only very few finer 

 forms were seen, and none very fine. Mierosclcra; these are of three forms, three-toothed isancorse 

 spatuliferse of two forms, and sigmata. 1. The large an corse have a slightly curved shaft, and in 

 either end three lanceolate teeth ; in both ends a rather narrow ala is found on either side of the 



