256 THE VOYAGE OF H.M.S. CHALLENGER. 



tolerably smooth, exhibiting four to eight protruding bosses on the middle of the axis rod, 

 and bear bell-shaped umbels with somewhat divergent rays of a flat paddle-like form 

 (PI. L. fig. 8). Others, measuring O'l mm., have a thinner, knotted axis rod, and umbel 

 rays with somewhat pointed ends, while small forms also occur, measuring 0'06 to 0'04 

 or even 0-03 mm. in length (PI. L. figs. 10, 11, 12). 



Very similarly constituted are the spicules which form the skeleton of the other, or 

 gastral side of the curved body. Here also the same oxypentacts occur as hypogastralia, 

 the same pentact pinuli as autogastralia, and amphidiscs of similar form and equally varied 

 dimensions (PI. L. fig. l). 



In the lateral wall of the larger efferent canals there are also oxji^entacts similar to the 

 hj'podermal and hypogastral forms, though less strongly developed and less long. Pinuli, 

 however, are absent, but the very abundant amphidiscs exhibit the same size and form as 

 those of the skin. 



The long lank marginalia which form the marginal fringe round the edge of the whole 

 sponge, usually measure some mm. (up to 1 cm. or more) in length. They are for the 

 most part quite smooth, form internally a long thin point, while externally they bear 

 small, somewhat distant lateral teeth, which project obliquely upwards and outwards. 

 They finally end externally in a slight club-shaped or bud-like swelling (PI. L. 

 figs. 1, 4). The long strongly developed spicules (5 to 10 cm. in length), which 

 project like a beard from the thickened lower portion of the sponge body into the mud, 

 are quite smooth. They end internally, i.e., in the sponge body, in a simple point, while 

 towards the lower outer free end they first decrease gradually in thickness, and then again 

 slowly increase, finally forming a double toothed very gently curved anchor. The two 

 teeth of the latter stand out almost at right angles from the shaft, are only slightly bent, 

 and end in a somewhat blunt point (PI. L. fig. 7). 



It is especially interesting to note that in the larger ca^dties of one of the specimens of 

 Polwpogon amadou some small, approximately spherical sponges were found, measuring 

 about 3 mm. in diameter, and undoubtedly young forms of Poliopogon. Since their 

 tissue was still in tolerable preservation, it was possible to make sections thi'ough the 

 small bodies. These results were important not only for the species but for the whole 

 genus, and indeed for the family of Hyalonematidse. 



I have given in PI. L. fig. 2 a diagrammatic representation of the structure exhibited 

 in successful axial sections of the young sponge forms. The characters more or less 

 distinctly prominent on various sections have been combined in a synthetic figure. 

 A central cavity with which all the diverticula of the folded chamber-layer directly 

 communicate, opens to the exterior at the upper pole of the somewhat transversely oval 

 section. This external aperture is not, however, completely free and open, but seems still 

 to be covered by a delicate membrane. The membrana reticularis which forms the 

 chamber-like diverticula, is perfectly continuous, forming a much folded and puckered 



