2 6 PORIFERA. II. 



iii the more narrow areas more or less transversely from one side of the furrow to the other. The size 

 of the pores is generally 0-035 — o-i mm . The thin pore membrane is not directly supported by the skeleton, 

 and only microsclera are found in it, especially chelae, partly in beautiful rosettes, partly scattered, but 

 also sigmata and rhaphides, singly and as trichodragmata. As mentioned by Ridley and Dendy 

 under Esperella murrayi (1. c. 67), a fine longitudinal striation is also found here in the strings of tissue 

 between the pores, which, as supposed by the mentioned authors, is perhaps owing to muscle-like 

 fibres. Frequently no pores are seen in the membrane of the furrows, but then it is to be supposed 

 that they are closed. Oscula are found in the upper end of the sponge, and sometimes some way 

 clown the sides; they are formed like small cones, of a height of only a few millimetres. The oscular 

 aperture is found at their point, and has a diameter of 1 — 2 mm . The wall of the oscular cone has a 

 dense spiculation of needles placed parallel to the longitudinal axis of the cone; these needles are of 

 the same form as those supporting the dermal membrane, that is to say of the smaller form. On the 

 upper part of the sponge, where oscula are especially found, the pore furrows are wanting or are only 

 found to a slight degree. 



The skeleton is of the dendritic type; from the base fibres issue which anastomose and branch, 

 most frequently more or less irregularly, up through the sponge. The real, single fibres are generally 

 not particularly thick, at most about o-47 mm ; but especially in the lower part of the sponge several 

 fibres are often united into strings apparently forming a single fibre, and reaching to a considerable 

 thickness, but under the microscope they are seen to be formed of several close-set fibres. Towards 

 the surface the fibres branch copiously, and run as parallel fibres of a thickness of about o-o6 mm and 

 with a distance of 0-25 — o^o" 1 ™ towards the dermal membrane; here the spicules spread in a peni- 

 cillate way and pierce the membrane. Where the pore furrows are found, the fibres pass off to the 

 sides, so that the membrane in the furrow is not supported or pierced by spicules. No transverse 

 fibres are found, but between the fibres and their finer branchings spicules and bundles of spicules 

 are scattered irregularly and more or less densely. In the outer part of the skeleton, at the surface, 

 it becomes more regular by the fact that the ends of the fibres here run parallelly with fixed inter- 

 vals; some transversely placed spicules are here found interwoven between the fibres. These transverse 

 spicules, as well here as deeper in the sponge, are generally of a form differing somewhat from that 

 of the spicules forming the fibres; they belong to the shorter and thicker forms, and are always a 

 little curved. The spicules in the outer eud of the fibres, which are spread in a penicillate way, con- 

 sist of styli of a definite kind, smaller than the other styli of the skeleton, as has already been 

 observed by Carter. Spongin is found in the fibres uniting the spicules; but nevertheless the fibres 

 are loose and little capable of resistance; with regard to this fact, however, there is some difference 

 between different individuals. 



Spicula: a. Megasclera are styli or slightly marked subtylostyli. They are of two forms, as the 

 spicules that support the dermal membrane spread in a penicillate way, as mentioned, are smaller 

 and of a form somewhat different from that of the skeletal spicules. These spicules supporting the 

 dermal membrane may, for the sake of shortness, be called dermal spicules, although they cannot be 

 said to form any particular dermal skeleton. The skeletal styli are straight, or have a larger or 

 smaller curve nearest to the upper end; this end is rounded and sometimes slightly swollen. The 



