664 PROF. SCHULZE ON THE STRUCTURE AND ARRANGEMENT 
each opening corresponds with a dimple-like depression on the external surface 
of the sponge. Between each four apertures which bound each rhomboidal 
field, a more or less rounded knob projects, which is separated from the nearest 
similar projection by shallow groove-like depressions, which correspond with 
the oblique connecting grooves of the wall-apertures (Pl. XVII. fig. 1). In 
many places, however, the bosses arranged in oblique series are fused together 
into more or less prominent ridges, sharp along the top and with tolerably 
steep walls,—the so-called “ combs,”—whose form and arrangement is so well 
seen in the macerated skeleton. 
I must, however, draw attention to the fact that the whole external surface 
of the tube, which shows a rather complicated relief, possesses no openings 
visible to the naked eye except these wall-apertures, but is covered by a con- 
tinuous connecting membrane. Every small irregular roundish darker mark 
which we notice in strong light corresponds to a subdermal hollow space which 
shines through this thin membrane (PI. XVII. fig. 1). The inner surface of the 
tube-wall does not appear so uneven. Between transverse annular beams of 
spicules, which project only slightly towards the interior, there are flat groove- 
like depressions, which divide the still less prominent longitudinal beams of 
the siliceous network into square ares of nearly equal size. The are thus 
separated by bounding ledges of different heights show, alternating tolerably 
regularly, either regular gaps perforating the tube, or less regular roundish 
excretory openings of cecal spaces in the wall of the sponge (Pl. XVII. fig. 2). 
Here and there deviations from the regular alternation of the two kinds of areze 
appear in the longitudinal and transverse rows, but these are only isolated, and 
occur usually where one of the longitudinally directed bands of siliceous fibres 
divides towards the broader upper end of the tubes, leading to the formation 
ef a new longitudinal row of ares, by which the symmetry of the structure is 
somewhat disturbed; whilst the ares, furnished with wall-openings, only show 
round the iris-like margin of these central openings a simple circle of small 
roundish excretory pores, of short water-ducts; the areee alternating with these 
show a somewhat important differentiation, as they sometimes contain only one 
or two, rarely three, large round openings of well-marked exhalent canals, 
sometimes a larger number of smaller openings of shorter wall-canals. We 
find that the areze with the larger ducts always lie under a comb, whilst those 
furnished with numerous smaller openings correspond to the boss-like pro- 
jections which are found where there isno comb. We find accordingly the 
areee with the large openings arrayed in oblique rows, which correspond with 
the combs (Pl. XVII. fig. 2). 
The soft parts are feebly developed on the cribriform plate, more or less 
arched, and with large irregular meshes, which closes the upper end of the tube. 
At most the layer of soft matter is developed comparatively thickly here and 
