4 8 



PORIFERA. II. 



here somewhat varying. The greatest thickness was measured to about 2 ,nm . When the layer is thick 

 it is frequently not of equal thickness all round, but the axis may be placed very eccentrically. In 

 the thickened part of the stalk the thickening is often chiefly due to the outer layer, but the axis 

 itself may also be thickened and the layer comparatively thin. The layer is quite compact showing 

 no canals, and is easily separated from the axis itself. The skeleton of this layer consists of exceed- 

 ingly densely interwoven, winding and finely spinulous tylostrongyla, which, although the layer contains 

 no spongin, are only to be separated with great difficulty. The spicules of the other parts of the 

 skeleton are united by a mass of spongin, quite clear under the microscope and consequently only to 

 be seen with difficulty. On a transverse section the axis is seen to be slightly yellowish, while the 

 sponginless coating of the stalk is whitish, and, when dried, gets a quite snow-white colour. 



Spicula: a. Megasclcra ; these are the spicules, partly of the skeleton proper, partly of the stalk- 

 coating. The spicules of the skeleton are of two forms: styli, chiefly forming the skeleton of the axis, 

 and subtylostyli, forming the skeleton of the branches, but also occurring in the axis. The styli of 

 the axis may again be divided into two groups: long, slender, most frequently straight ones, and short 

 thick, and curved ones; these two groups, however, are not sharply separated, but connected by tran- 

 sitional forms, while there seem to be no transitions between the styli and the subtylostyli. The styli 

 are fusiform and evenly tapering towards both ends; in the rounded end, moreover, a short, rather 

 abrupt tapering is found, by which the styli are still more easily to be distinguished from the sub- 

 tylostyli; this tapering is most marked in the thick forms. The other end has a short, somewhat 

 stubby point. The length is o-68 — i mm , the thick ones generally not exceeding o-875 m,n , and sometimes 

 going down to ca. o-6 mm . The thickness varies altogether from 0-015— o-032 mm ; for the two forms it 

 may be stated to be about 0-015 — cvo2i mm , and 0-021 — o - 032 ,mn . Of the styli the thicker ones, to be sure, 

 are found throughout the axis, but they are especially numerous towards the base. The subtylostyli 

 are slender and straight, the head is only little marked and is placed a little below the rounded end. 

 The point is short and often almost stubby. They are fusiform, tapering a little towards either end. 

 The length varies from 0-52 — 075 mm , and the thickness in the middle is 0-009— O'oij" 1 "'. As mentioned 

 above, the subtylostyli form the skeleton of the branches, and are also found in the stalk, especially 

 in the outer layer. The spicules of the coating of the stalk are tylostrongyla, minutely and densely 

 spinulous; they are sinuous and curved in very different manners. The head is rather distinctly marked, 

 sometimes it is placed a little in on the needle; the other end is broadly rounded, and the needle is of 

 about the same thickness throughout its length; sometimes it tapers a little towards the end. The 

 length is between 0-05 and o-i37 mm , the thickness was measured to o-ooi — o-oo28 mm . The thinnest forms 

 that have been observed, thinner than o-ooi mm , are so finely spinulous, that the spinules are almost not 

 to be observed, and the very finest ones are perhaps quite smooth; I suppose them to be developmental 

 forms, b. Microsclcra ; these are anisochelse palmatse of two forms and sigmata. 1. The charac- 

 teristic anisochelae are small and of the structure peculiar to the subgenus. The shaft is curved. 

 The two ends are exceedingly different; the upper end is provided with very large alse folding round 

 on the side, the tooth is considerably shorter and narrower. When the chela is seen from the side, 

 the axis is seen to bend round below like a hook, and alse and tooth are present in the common way; 

 these parts are very narrow, but they are only to be seen with extreme difficulty, and so it is not 



