io2 PORIFERA. III. 



shells of Bivalves, Hydroids, and once it is recorded (Topsent 1S92) as on an Inackus. The 

 greatest extent to which the sponges in my material ordinarily reach is determined by the size of the 

 Valdhciiiiia, and is thus about 20 mm , and the incrustations are as a rnle very thin, scarcely reaching 

 0'5'" m . The specimen on the stone has an extent of 25 mm . The colour (in spirit) is generally 

 pale yellow, sometimes deeper yellow or brownish. Topsent records (1. c. 1888) that it may sometimes 

 be violet. The surface is smooth. The dermal membrane is delicate and transparent and without 

 spicules; it is thin, but when it remains on the sponge in its normal position it is not at all fragile, 

 and is easily separable; but in most cases the membrane is more or less destroyed, either quite 

 wanting or remaining only as patches or rags; probably the membrane is in the living sponge soft 

 and hence easily torn, but on hardening in alcohol it becomes much tougher. Topsent (1. c. 1888) 

 speaks of "La peau epaisse" but he is here evidently thinking of the whole tissue occupied by the 

 dermal spicules; in 1892, on the other hand, in the description of the specimens of Myxilla radiata 

 which in 1904 he declares belong in reality to H. Ditjardinii, he describes a thin dermal membrane. 

 Oscula and pores: Bowerbank says: "Oscula and pores inconspicuous", and Topsent (I.e. 1888) says: 

 "Les orifices aquiferes sont petits". Oscula I have not observed, but pores I observed on the other 

 hand in those specimens in which the dermal membrane was in good condition; the pores are some- 

 what interesting; they are gathered in beautiful sieves which may be present in rather great numbers; 

 the sieves are generally oval and are seen as slight, somewhat sharp-bordered impressions inclosing a 

 sieve-like membrane; they are not very conspicuous and require a close examination to be detected, 

 and they are best rendered visible when the sponge is a little dried. The sieves have generally a 

 largest diameter of 08 — 2 mm ; the pores are dense-lying, more or less oval and of a diameter of 

 0-028— o-o8 mm . 



The skeleton. The dermal skeleton consists of fibres, bundles or more loosely scattered spicules 

 which stretch from near the base and up to the dermal membrane; it may be somewhat differently 

 developed in different places, in some places it consists only of scattered spicules together with some 

 few bundles, while in other places the spicules and bundles are much more crowded; sometimes also 

 the spicules may form rather long fibres running horizontally below the dermal membrane; finally, as 

 already pointed out by Topsent, the dermal skeleton is naturally most developed in the thickest 

 specimens, as it alone occupies the space between the basal skeleton and the surface. The main 

 skeleton consists of vertical acanthostyli with their heads placed on the substratum; they are some- 

 what distantly arranged; at the base there is a distinct amount of spongin forming a more or less 

 distinct basal lamella, and the spongin also stretches somewhat up along each acanthostyle, thus 

 imbedding the lower part of it and forming a distinct coat. 



Spicula: a. Megasclera. 1. The skeletal spicules are acanthostyli; they are straight, only 

 rarely slightly curved; the head is globular, sometimes distinctly marked off, at other times less so on 

 account of the shaft being rather thick below, but also in this latter case of a globular appearance; 

 the shaft tapers evenly from the head to the point. The styli are spined in their whole length, only 

 the spines are sometimes few and dispersed towards the point, and a short outermost part may then 

 be smooth. The spinulation may vary much in different specimens being more or less dense; the 

 spines are medium sized and reclined on the shaft, but radiating straight out or curved somewhat 



