EURETE ERECTUM. 127 
The colour in spirit is light yellowish brown. When the tube-wall is ob- 
served by transmitted light, numerous small dark brown spots, about 1 mm. 
apart, make their appearance in it. These appear to be accumulations of deep- 
sea ooze in the bottoms of wide, vertical, sacular canals which lead from the 
outer surface into the deeper parts of the tube-wall. 
Canal-system. The flagellate chambers (Plate 30, figs. 7c, 10c, 17c) are 
spherical or short oval, and measure 60-80 u in diameter. 
Skeleton. A special dermal and a special gastral skeleton are developed 
besides the internal. The internal skeleton consists of a supporting network 
and loose spicules; the dermal and gastral skeletons are exclusively composed 
of loose spicules. 
The supporting skeleton-net (Plate 30, figs. 4-6, 10-12, 17; Plate 31, fig. 24) 
appears as a lamella corresponding in shape to the tube-wall, but thinner than 
this. It is composed of smooth beams, 30-105 yu thick. In its outer part 
(Plate 30, fig. 4) the meshes are irregular, mostly triangular, the larger ones 
generally a little under 200 » wide. Its inner part (Plate 30, figs. 6, 11) is more 
regular, composed chiefly of longitudinal and transverse beams enclosing square, 
rectangular meshes, mostly 370-400 uw long, and 170-400 » broad. Here and 
there small hexactines, 80-120 » in diameter, are attached vertically to the beams 
of the net by one of their rays. From both faces of the lamella formed by the 
skeleton-net large thorns protrude. These thorns arise from the superficial 
nodes of the net, point outwards, and are nearly vertical or, more rarely, oblique 
to the surface. They are straight or slightly curved, and quite regularly conic, 
pointed or, rarely, inflated at the end, and covered with broad and low, terminally 
rounded spines, which decrease in size distally. The thorns on the outer, dermal 
side (Plate 30, figs. 7e, 10e) are mostly 140-340 y» long, and 20-50 uw thick at the 
base. The thorns on the inner, gastral side (Plate 31, fig. 24g) are larger, 230- 
430 u long, usually 270-400 », and 35-60 wu thick at the base. 
The loose spicules of the choanosome are uncinates and discohexasters. The 
former are fairly abundant, the latter rather scarce. 
The dermal skeleton is composed of hexactine pinules and small scopules. 
The dermal pinules are very numerous and form a continuous layer on the outer 
surface. Their lateral rays (Plate 30, figs. 7a, 10a, 12a, 17a; Plate 31, fig. 22) 
extend paratangentially and together form a network, usually with more or less 
quadratic meshes (Plate 31, fig. 22). Their proximal and distal apical rays are 
situated radially (Plate 30, figs. 7d, 10d, 12d, 17d). Their centres are on an 
average 130 » apart. The dermal scopules are situated radially. Most of them 
