8o 



PORIFERA. I. 



The canal system. In the outermost part of the rind subdermal cavities are found, often as 

 horizontal canals into which the pores lead, and from these cavities canals go in through the rind; 

 they are rather numerous, so that a great many round holes, generally of a diameter of i — 3™'", are 

 seen on the inside of the rind-laj'er, when this layer is separated from the inner tissue"). These canals 

 are lined by a thin membrane the spicules of which are gathered into bands or fibres running parallel 

 with each other, sometimes in the longitudinal direction of the canals, sometimes obliquely on, or 

 across this direction. The canals continue through the interior body, the direction apparently being 

 chiefly towards the centre; in the soft tissue they are surrounded by a membrane, so that, when the 

 tissue is washed away, they appear as membranous tubes. This membrane is provided with a spicula- 

 net of loose fibres, and innermost it is lined with the membrane mentioned above. Other canals, 

 presumably the excurrent ones, come together and open into the fistute, in the base of which they 

 converge, and thev continue in the inner cavity of the fistulse. The walls of the fistula; are provided 

 with a dense plait of fibres; the wall is easily separated into two lamella, of which the inner one is 

 provided with the same kind of spicula-net of loose fibres as is found in the membrane surrounding 

 the canals. The fistulse are inmost lined with the same membrane as that lining the canals, and it 

 is provided with sjsicnles in the same manner. 



The skeleton. In the rind-layer the skeleton consists of a network of rather thick, i5oly.spicular 

 fibres forming irregular, most frequently polygonal meshes (PI. XV, fig. 2). The thickness of the fibres 

 is somewhat varying, but generally it is from 0'05 — o-i"™. The meshes of this network vary ver)- 

 much in size in different places, and tliey are smallest inward; in these meshes are seen thinner poh- 

 spicular, or quite unispicular fibres with an irregular course, and also forming an irregular network. 

 In the network no distinction can be made between fibres running towards the surface, and fibres 

 running parallel to it; here and there longer and most frequently a little thicker fibres are seen, but 

 they have also a quite irregular course. The spicules of this network are cemented by a clear, not 

 copious mass of spongin; it is especially observable where it unites the ends of the spicules, but in 

 the fibres it unites the spicules in their whole length. Outermost a layer of spicules are found at 

 right angles to the surface, and projecting a little, so that the surface of the sponge becomes slightl}- 

 shaggy; these spicules frequently show a somewhat fanlike arrangement. Between the perpendicular 

 spicules others are interwoven partly oblique to, partly parallel to, the surface; therefore the skin, 

 when seen from the surface, shows a rather dense, irregular network of spicules intercrossing in all 

 directions, and from which needles rise in a vertical direction. The dermal membrane rests on this 

 network; it cannot be separated bv itself, but the whole layer with the x'ertical needles is easily peeled 

 off. This network is also cemented by a mass of spongin, not only found at the ends of the spicules, 

 but in all places where the spicules touch each other. Inwardly the skeleton of the rind is absolutely 

 sharply boimded from the tissue inside of it. In the interior bod}- the skeleton onh- consists of scat- 

 tered needles without any visible order; they impart no firmness to this tissue, neither is an)- spongin 

 found here. As mentioned above, the canals crossing the interior body, are surrounded by a membrane; 

 this membrane has a skeleton of very loose fibres forming a network, the longest fibres of which run 

 in the longitudinal direction of the canal (PI. XV, fig. 3); also in this network the spicules are united 



•) It is these holes which are called oscula by Bo werb auk 1. c. 304, as he had only fragments of the sponge at that time. 



