PORIFERA. III. 



79 



This species greatly resembles the preceding one in external appearance. One specimen is growing 

 on a stone, which it has nearly quite overgrown, its greatest extent may be estimated to 22 mm ; three 

 other specimens grow on a stone together with specimens of Histoderma physa, Hymedesmia nummulus, 

 H. filifera and Plocamia sp.; the largest of these specimens is i2 rara in greatest extent. The thickness 

 of the sponge is about 0-5 mm . The colour (in spirit) is whitish with a slightly bluish, somewhat milky 

 tint, but in places it may be shaded brownish on account of a film-like covering, such as mentioned 

 in the preceding species. The surface is smooth. The dermal membrane is thin and has no proper 

 skeleton; it is of the same curious, skin-like consistency and appearance as in truncata, and it has 

 very probably been mucous in the living state of the sponge. Oscula and pores: with regard to the 

 oscula the facts are quite as in truncata; on the largest specimen there is a cylindrical, rather large 

 oscular papilla about in the middle, it has a length of 6 mm . Pores were not observed. 



The skeleton is constructed quite as in tnmcata. The dermal skeleton consists of somewhat 

 scattered bundles of spicules, the bundles generally consisting of rather many spicules; at the base of 

 the oscular papilla the bundles form fibres which stretch up in the wall of the papilla, here forming 

 a dense spiculation of close-lying, parallel spicules. The main skeleton consists in the ordinary way 

 of erect acanthostyli placed with the heads on the substratum; the spicules are rather close-standing. 

 So far as I could observe there is no spongin present. For the rest the skeleton in most places shows 

 all the same conditions and alterations as described above under truncata, and I think these conditions 

 are due to the same factors as suggested for this species. Thus the present species also gives the im- 

 pression of being strongly contracted, and the dermal membrane is obviously wrinkled and folded, 

 and moreover it also shows numerous cells with refringent granules. 



Spicula: a. Megasclera. i. The skeletal spicules are acanthostyli of a very characteristic 

 shape; they may to some slight degree remind one of the discasters in Latrunculia. They are short, 

 thick and regularly conical; they are strongly spined, and the spines are large, radiating hori- 

 zontally, not reclined; there is no head-swelling, but there is a slightly thinner and unspined part 

 above the base, forming, as in truncata, a neck-shaped constriction, and this constriction makes it 

 seem as if a head-swelling were present; the constriction mentioned and the outermost point are 

 the only smooth parts. The length is very uniform, 0-065 0x571 mm and the diameter at the base is 

 0-022 0-027 mm . 2 - The dermal spicules are tylota, but the swellings are often so small that they 

 are nearly strongyla; they are straight with a cylindrical shaft of equal thickness in the whole 

 length; sometimes they are very slightly polytylote. The length is 0-30 O'4O mm , and the diameter is 

 0-008 0x513". b. Microsclcra; these are chelae arcuatse; they have a slightly curved shaft, the alae 

 are lobe-shaped and the tooth elliptical; the lower edge of the alee is somewhat incised when the 

 chelae are seen in front view. The length is 0-023 0-028 mm and the diameter of the shaft 0-003 

 0-005'". The chelse may vary somewhat, being more slender or more robust, and some of them are 

 found showing a very robust shape, giving the impression of being not normal but influenced by 

 abnormal deposition of silica. The chelae occur especially in the dermal membrane, and on the 

 oscular papilla they are present in great numbers. 



This species is related to the two preceding, but it is characteristically distinguished, especially 

 by its acanthostyli, but also by several other characters. 



