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 GO-80 m 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 fi thick. In its outer part 

 (Plate 30, fig. 4) the meshes are irregular, mostly triangular, the larger ones 

 generally a little under 200 m 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 m long, and 170-400 m 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, lOe) are mostly 140-340 ^ long, and 20-50 ix thick at the 

 base. The thorns on the inner, gastral side (Plate 31, fig. 24g) are larger, 230- 

 430 II long, usually 270-400 fi, and 35-60 /x thick at the base. 



The loose spicules of the choanosome are uncinates and discohexasters. Tlie 

 former are fairly abundant, the latter rather scarce. 



The dennal 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, lOd, 12d, 17d). Their centres are on an 

 average 130 n apart. The dermal scopules are situated radially. Most of them 



