PORIFERA. II. j,. 



1S93. Myxilla incrustans Levinsen, Det vidensk. Udbytte af <Hauch's> Togter, 419, 17, Tab. I, 



Fig- 3 1 — 34- 

 1896. Dendoryx incrustans Topsent, Resultats scient. de la Campagne du Caudan», 274, 15. 

 1896. Myxilla incrustans Lambe, Transact, of the Royal Soc. of Canada, Ser. 2, II, Sect. IV, 191, PI. I, 



figs. 10, 10 a — d. 



Incrusting, forming irregular, massive masses, or being more roundish lumpy. The surface with 

 sinuous grooves, more rarely even, not shaggy. The dermal membrane a thin film supported by peuieil/afe 

 bundles of dermal spicules. Oscula scattered, sometimes on the top of low cones. The skeleton chiefly a 

 polyspicular reticulation of triangular meshes, sometimes more irregular. Spicula: Megasclera: the skeletal 

 spicules acauthoslyli with, scattered spines o-ig — o-jj""", the dermal spicules tornota with slightly spined, 

 rarely smooth ends cny — 0-26"""; microsclcra of three forms, aucorcc spatulifene of two sices, large ones 

 o-ojS—o-O//""", small ones o-oij — 0-028, sigmata 0-024 — 0-075""". 



This species may vary rather much in appearance, and in this respect it recalls Halichondria 

 panicea. As in this species, however, the form is highh dependent on the substratum on which the 

 sponge is growing. The most frequent forms are more or less extended, incrusting masses that may 

 be thinner or thicker; then it may rise to cushion-shaped bodies or to irregular, roundish lumps. 

 When growing on Algae, roots of Algae or similar bodies, it may have a quite irregular form, but then 

 it is most frequently only incrusting. Oscula may sometimes be found on more or less distinct, conical 

 projections on the cushion-shaped or lump-shaped sponge. The species seems sometimes to be growing 

 directly on the sea-bottom ; a few of the specimens in hand are not attached, but their basal surfaces 

 are filled with sand and gravel. According to Johnston and Bower bank, it is also frequently 

 found on rocky bottom, growing on the rock. It is very often growing on P^Arw-species, and then it 

 is most frequently rather regularly cushion-shaped or flatly semiglobular. In the Iugolf-material we 

 have it on Pecteu islandicns ; Bowerbank mentions it on Pecfeu operations, and Lambe has it from 

 the American coast on Pecteu tenuicostatus. Levinsen mentions it from Denmark on Mytilus modiolus 

 and Cyprina islaudica. As rather thin, irregular incrustations it is found on barnacles, Bryozoa, worm- 

 tubes, and Lamiuaria-roots. The largest specimen in hand, which is not attached, has a greatest 

 breadth of ii5 mm and a height of 90 mm ; most of the specimens are not so large and especially not so 

 high; a middle-large one, growing on a Pecteu, is 75 mm broad and ca. 25 mm high. The smallest speci- 

 men in hand forms a small crust on a barnacle, it has a greatest extent of 9 mm and is only o'5 mm 

 thick. The consistency is of middle hardness, it is only little elastic and rather fragile. The colour 

 (in spirit) is most frequently light yellowish; in the fresh specimens it is described as yellow to orange. 

 The surface may be somewhat varying; it may sometimes be quite even and smooth, but most fre- 

 quently it is more or less, often highly grooved, and the grooves are separated by winding and wrin- 

 kling walls. These walls are formed by the parts of tissue separating the canals; these canals are 

 present in large numbers, they run very close to each other, and are most frequently directed towards 

 the surface. Although the dermal membrane, as will be mentioned later on, is supported by the 

 projecting spicuia-bundles, the surface is nevertheless almost smooth, the spicules projecting so little, 

 that they are only to be observed by means of a highly magnifying lens. When the walls are more 







