PORIFERA. III. 



77 



28. H. truncata n. sp. 

 PL III, Fig. 9, PL VIII, Fig. 6. 



Incrusting, thin; stirface smooth, bearing some small cylindrical or conical oscular papilla with 

 a dense skeleton of dermal spicules in the wall. Spicula: megasclera; the skeletal spicules short, some- 

 what club-shaped, strongly and entirely spined acanthostyli, truncate at the apex and vuith a neck- 

 shaped constriction above the base; they are of one size 0-065 a 077 mm ; the dermal spicules tylota 0-28 

 o-j6p'" m ; microsclera chela arcuatcB 0-021 0-023""". 



Most specimens of this species grow incrusting on stones, one specimen on a Brachiopod-shell 

 and one on a fragment of a mussel-shell; the specimens are of different sizes, the largest reaches an 

 extent of 40"""; the thickness is very small, generally scarcely exceeding 0-2 mm . The colour (in spirit) 

 is light bluish white, somewhat milky. The surface is entirely smooth without projecting spicules. The 

 dermal membrane is thin and transparent, it is generally easily separable, and seems to be without 

 spicules, resting on the skeleton beneath, but the facts with regard to the dermal membrane are not 

 easily understood; the membrane has a curious, skin-like appearance and consistency, and it is 

 generally filled with cells containing somewhat refringent granules ("cellules spheruleuses"?), and I 

 think the mentioned state of the membrane is due to the influence of alcohol; in most of the speci- 

 mens there is moreover outermost a thin, brownish film, more or less easily separable, which is, I 

 think, formed by influence of the alcohol on some sort of mucus. Oscula and pores: In most of the 

 specimens there are some cylindrical or slightly conical papillae on the surface; they may reach to a 

 length of 3 mm ; these papillae are oscular tubes with the oscular opening at the summit, though in the 

 present state they are generally closed. The number of papillae present in the various specimens 

 varies from two to eleven. The two smallest specimens showed no papillae. Pores were not observed. 



The skeleton. The dermal skeleton consists of bundles of dermal spicules, which stretch from 

 the main skeleton, or almost from the base, and up towards the dermal membrane; they are thus lying 

 obliquely or more or less horizontally. In some places the bundles are more numerous than in others, 

 but most frequently they are scattered and not at all numerous; the bundles are rather thick, consisting 

 of numerous spicules. Near the oscular tubes the bundles form fibres which run from various sides 

 into the oscular papilla and form in the wall of the papilla a dense layer of spicules lying parallel to 

 the longitudinal axis of the papilla. The main skeleton: In most places the main skeleton is con- 

 structed in the ordinary way, the acanthostyli are erect, with their heads based on the substratum; 

 they reach from the base to the dermal membrane, but do not project beyond it; they are, at all 

 events in most places, rather close-standing. I could not detect any spongin. Such is, as said, the 

 structure of the skeleton in most places, and I think these are the places where the sponge is quite 

 undisturbed; but in many other places this is not so, the acanthostyli may here be lying down 

 and crossing each other in all directions, and, what is the most remarkable fact, they may be lying 

 more or less horizontally in or near the dermal membrane; in most places they are then crowded 

 together, in others they are much more scattered. Whether this condition is normal or not, I am not 

 able to say with certainty, but I think it is due to contraction, especially as the normal Hymedesmia- 

 arrangement of the skeleton is seen in many places. It was stated above, that the dermal membrane 



