PORIFERA. III. 



123 



This species grows on a large stone together with a Sarcophyton and various sponges of the 

 genera Tedania, Hymcdesmia and Crella. It forms an incrustation of a greatest extent of 30 mm , the 

 thickness being scarcely 0-5 mm . The colour (in spirit) is greyish. The surface is smooth without pro- 

 jecting spicules. The dermal membrane is not especially thin and it is somewhat solid; it is charged 

 with microsclera, especially sigmata, and there are fibres of dermal spicules in it or just below it. 

 Oscnla and pores were not observed. 



The skeleton. The dermal skeleton consists of bundles and fibres stretching up to the dermal 

 membrane, but in a very horizontal direction, and especially there are fibres running almost or quite 

 horizontally in or just below the membrane; the dermal skeleton is thus chiefly constructed of hori- 

 zontal fibres, and the fibres may reach some length; otherwise they have a more or less irregular 

 course, and they are often curved; they may be of different thickness but are often rather thick, e. g. 

 o-io mm . The main skeleton consists of acanthostyli placed with the heads on the substratum and 

 directed vertically, or generally more or less obliquely upwards; they are much scattered and present 

 only in somewhat small number; the main skeleton is thus diffuse and irregular and on the whole 

 somewhat weakly developed. The acanthostyli are not evenly scattered over the surface of the sub- 

 stratum, but they are to some degree collected in bundles with a few spicules in each. There seems 

 to be a small amount of spongin at the base of the acanthostyli. 



Spicula: a. Megasclera. 1. The skeletal spicules are acanthostyli which are divided into 



two groups, large and small; the large styli are straight or a little curved; the basal end is rounded, 



without any head-swelling or with this only very weakly developed, the other end tapers into a 



middle-long apex. The styli are somewhat densely spined in almost their whole length, only a short 



apical part being smooth; the spines are of medium size or small, and they are not reclined but 



radiating straight out; on the basal part there may sometimes be some stronger spines. The length 



of the large styli is 0-42— 0-53 mm and the diameter at the base 0-017 — o-024 mm . The small styli are 



straight; they have generally a somewhat distinct but small head, and the point is somewhat short; 



the spinulation is continued somewhat beyond the middle so that the apical part is smooth. The 



length is 0-089 — o - 2i n,m and the diameter at the base o - oio — o - oi4 mm . The small acanthostyli are of 



rather scarce occurrence. 2. The dermal spicules are tylota; they are straight or sometimes slightly 



curved, the shaft is cylindrical, not thickened in the middle; the ends have small but somewhat 



distinct swellings; one end is a little thinner than the other and has the swelling more suddenly 



marked, but this feature is only slightly pronounced and only noticed by close examination. The 



length of the tylota is 0-38 0-50 ,nm and the diameter of the shaft 00046— 0-007 mm . b. Microsclera are 



of four forms, chelae arcuata, sigmata and forcipes of two forms. 1. The chelae have an evenly 



curved shaft, the alas are lobe-shaped, somewhat narrow and deeply incised below, the tooth is 



elliptical, of the same length as the alae. The chela varies much in size, the length is 0-014 — 0-038 mm 



and the diameter of the shaft o-ooi— 0-004 " ,m . The intermediate sizes are not numerous and hence the 



chelae give the impression of being divided into two groups. 2. The sigmata are large, of common 



shape and more or less contorted; they vary somewhat in size, the length is 0-077— o-i6 mm , an d the 



thickness is 0-0040 — o-oo8 mm . 3. The large forceps is of the ordinary hair-pin-like shape with legs 



of equal or about equal length; they have a round curve above, and the legs are somewhat divergent; 



16* 



