SIDONOPS BICOLOR. 47 



found on the more sheltered parts. Incrusting symbionts, desmacidonid 

 sponges, two species of Bryozoa, etc., grow on the afferent portions of the surface 

 of most of the specimens, while the efferent areas are free from symbionts. 



The colour of the surface varies in these spirit specimens from whitish to 

 reddish or purple-brown; some parts of it are, as mentioned above, usually 

 much lighter in colour then others, the under side appearing to be less pigmented 

 than the upper side. Occasionally, particularly in the specimens from Station 

 4420, I have noticed that the margins of the efferent pores are somewhat lighter 

 in colour than the adjacent parts of the surface. The interior is dirty brownish 

 or greenish white. 



The superficial parts form a cortex (Plate 9, figs. 15-17) which contains 

 a sterraster-armour 0.9-1.8 mm. thick. The sterrasters do not always extend 

 down to the choanosome, a thin fibrous layer often intervening between them 

 and the latter. This layer, which is composed of paratangential fibres similar 

 to those connecting the sterrasters, is more clearly made out in one of the speci- 

 mens from Station 4420 than in the others. In the darker parts of the cortex 

 pigment cells are observed. These contain large spherical granules, brown in 

 colour, which stain deeply in azure. The number of these granules in each cell 

 is not great. On or just below the outer surface the pigment cells are very 

 numerous and often form a continuous layer wliich has the appearance of an 

 epithelium. This I observed chiefly in a specimen from Station 4420. These 

 cells are here massive, or somewhat elongated, irregular in outline, about 10 /i 

 broad and 12-25 /< long. Pigment cells also occur in the lower parts of the 

 cortex, within the sterraster layer, but here they are long and slender, and 

 arranged radially around the sterrasters. This shape and position of the deep- 

 l}ing pigment cells are apparently due to the position of the connective-tissue 

 fibres which radiate from the sterrasters and between which they lie. 



In the choanosome of a specimen from Station 3168 I found numerous oval 

 bodies 20-35 fi long and 10-20 /< broad which consist of a nearly hyaline sub- 

 stance uniformly staining with haematoxylin and azure. In these bodies 

 neither an enveloping membrane nor a nucleus could be detected. Most of 

 them are densely crowded in band-Uke zones, some isolated and scattered. 

 Similar bodies, scattered singly throughout the choanosome, have also been 

 observed in a specimen from Station 4420. 



Canal-system. In many of the specimens I have been able to make out the 

 afferent pores. These are, as stated above, chiefly distributed over the convex 

 parts of the surface and arranged in more or less circular groups (Plate 10, fig. 15) 



