PORIFERA. III. 89 



with horizontal spicules. Oscula and pores: the mentioned papillae are oscular and pore-papillse; in 

 outer appearance there is almost no difference between the two kinds, only the oscular papillae are 

 generally more conically pointed at the apex, while the pore-papillse are more broad here; the latter 

 are closed by a pore-sieve, while the oscular papillae have a simple opening in the summit. The pore- 

 papillse are more numerous than the oscular papillse. 



The skeleton. The dermal skeleton; the skeleton consisting of the dermal spicules forms the 

 greatest part of the whole skeleton; it consists of fibres and bundles which stretch in an irregular 

 way from or nearly from the substratum up to the dermal membrane; the course of the fibres is, as 

 said, irregular, and they are often more or less curved, and may thus be seen running somewhat 

 parallel with the surface; the fibres may be of very different strength, but often rather thick, up to 

 0-3 mra , and consisting of many spicules. In the dermal membrane the spicules lie horizontally, but 

 irregularly, crossing each other in all directions; they lie somewhat scattered, and the membrane is to 

 be seen everywhere between them; they lie thus much more scattered than in filifera, and not bundle- 

 like collected. The skeleton of the papillae is constructed quite as in filifera, and also here the 

 spicules cross each other regularly and under acute angles. The main skeleton is quite irregular and 

 scattered on account of the way in which the sponge grows on the substratum; it forms thick bundles 

 scattered everywhere between the foreign sponge-spicules and other particles of the substratum, and 

 always forming the lowermost part of a fibre of dermal spicules; the acanthostyli are thus not at all 

 evenly distributed at the base of the sponge, but very scattered and only present as bundles, from 

 which fibres of dermal spicules proceed. The bundles are generally large and consist of many 

 spicules, they may have a thickness of 0-5 mm . At the base of the bundles there is a distinct mass 

 of spongin. 



Spicula: a. Megasclera. i. The skeletal spicules are long and slender acanthostyli; they 



are straight or nearly so and evenly tapering, but the outermost point is short; they have no or only 



a very slight head-swelling and are somewhat densely spined at the base and some way out, but on 



the largest part of the shaft the spines are somewhat scattered; at the base and especially a little 



above it the spines are somewhat large, but for the rest they are small. The styli do not generally vary 



much in length, from 0-41 O75 mm with a diameter at the base of 0-021 0-026 mm ; it will be seen that 



there is thus no very great difference in size between the styli; some smaller ones may however be 



found, reaching only o-i8 mm in length, but these are very scarce and perhaps not always present. 



2. The dermal spicules are long strongyla; they are straight, sometimes slightly, irregularly curved, 



and they may be slightly polytylote; the ends may be very slightly swollen. The length is 0-48 



o-8o mra , and the diameter in the middle 0-007 0-013 mm . Microsclera; these are of two forms, chelae 



arcuatae and sigmata. i. The chelae have a curved shaft, the free middle part of which is about one 



third of the length of the chela, the alae are lobe-shaped, the tooth elliptical, pointed. The length is 



0-042 0-048 mm , and the diameter of the shaft 0-005 '7 """ 2 - The sigmata are of common shape, 



more or less contorted up to a quarter of a turn; they vary considerably in size, the length is 0-031 



o-O96 mra and the thickness relatively 0-002 0-006 mm ; the larger forms are the most common. The 



two forms of microsclera occur at definite places in the sponge; in the dermal membrane both chelae 



and sigmata are present in equal numbers; the chelae occur moreover in the wall of the papilla, 



The Ingoll-Expcditiim. VI. 3. 12 



