gg PORIFERA. III. 



The skeleton. The dermal skeleton; the dermal spicules form bundles which stretch almost 

 from the base of the sponge and upwards, thus occupying the whole space between the base and the 

 dermal membrane, lying in different directions between the erect styli of the skeleton. In the dermal 

 membrane the spicules lie horizontally, more or less densely, but quite irregularly, they lie singly 

 or here and there a couple together; the membrane is thus provided with a proper skeleton. The 

 skeleton in the wall of the papillae is formed of spicules lying parallel to the longitudinal axis of the 

 papillae and very densely, and it is formed both of the spicules lying in the membrane, and of those 

 stretching from the underlying skeleton up in the wall. Besides the mentioned skeleton the membrane 

 is somewhat densely charged with chelae which are specially numerous in the pore-membrane in the 

 papillae. The main skeleton consists of erect acanthostyli with the heads placed on the substratum; 

 the longest of them reach through the whole thickness of the sponge up to the dermal membrane. 

 At the base there is a not very conspicuous amount of spongin. 



Spicula: a. Megasclera. r. The skeletal spicules are acanthostyli; they are divided into two 

 well defined groups; the large ones are long and evenly tapering; they are slightly curved, the 

 curvature as a rule placed more or less near the base; the head is only slightly swollen, but, however, 

 somewhat distinct, it is densely beset with spines which are blunt; from the head and a little way 

 out there are spines, decreasing in size outwards, the remainder of the shaft is smooth, or has onlv 

 scarcely perceptible and scattered grains. The small acanthostyli are straight or nearly so; they have 

 a small head which, however, seems larger on account of its somewhat long, radiating spines; thev 

 are spined in their whole length, the spines on the shaft are reclined. The length of the long 

 acanthostyli is 0-47 — ri9 mm , and the diameter of the head 0-025 — °"°37 mm > the small styli are 0-119 — 

 0-26""" long, with a diameter of the head of 0-020— 0-025 mm . Small individual variations in size may occur. 

 Developmental stages of both sizes of the styli were seen in small numbers, showing that the two sizes 

 are essentially different from the beginning. 2. The dermal spicules are tornota with intermediates 

 to oxea; they are straight and long tapering; they vary a good deal in thickness, when they are thin 

 they may be termed tornota, but often they are more fusiform and are then oxea; their length is 

 0-34 — 0-44 mm with a thickness of 0-008— 0-013 mm . Besides these spicules there are some stronger, 

 especially thicker, and fusiform oxea of a length of 0-38 0-50""" and with a thickness in the middle 

 of 0-017— 0*028 mm . — With regard to these latter spicules the facts are somewhat curious; they are 

 mentioned both by Bowerbank and by Topsent, and both authors say, that they are present only 

 in small numbers. But they seem to be subjected to great variations with regard to the number in 

 which they are present; generally they are only found in very small numbers, and such is the case 

 in most of my specimens; but in one specimen they are very numerous; whether they are few or 

 many they are always only present in the dermal membrane, and lying horizontally, but they are not 

 found in the bundles which stretch down into the sponge; in the specimen mentioned, with numerous 

 thick oxea, the horizontal spicules in the dermal membrane are almost all of this kind; as the skeleton 

 in the wall of the papillae is formed both of the spicules lying in the membrane itself, and of those 

 belonging to the fibres stretching up into the wall, the skeleton of this latter consists consequently 

 outwards of thick oxea, but inwards of thin ones. In the specimen with numerous thick oxea these 

 are at the same time thicker and upon the whole larger than in the other specimens. — Develop- 



