REPOET ON THE TETRACTINELLIDA . 
29 
of a middle layer of myocytes, having collencytes associated with them, as in the oscular 
tube, and superficial layers of collenchyma containing similar cellular elements to those 
which occur below the epithelium of the oscular tube, excepting the subepithelial columnar 
and fusiform cells, which are here only to be observed rarely and as isolated examples. The 
muscle-fibres may be distinguished as main fibres and their branches, the former sur- 
rounding the larger oval or polygonal areas of the wall, the latter subdividing them into 
still smaller areas. In the centre of the smallest areas is a circular pore or short tube, 
which opens freely into the surrounding sinuses of the excurrent canals. Each pore is 
surrounded by concentrically arranged myocytes, which are crossed by fusiform cells 
running radiately (PI. XXXIX. fig. 4). The outer ends of these cells unite with the epi- 
thelium, the constituent cells of which are very clearly indicated by the outward dome-like 
bulging over the nucleus, or they may apparently sometimes project beyond, though I 
could by no means be sure of this, in a fine short bristle-like process. From the nucleus 
to its outer extremity, or about half the length of the cell, is a distance of 0'04 mm., as 
seen in portions of the wall viewed en face mounted in glycerine. In transverse sections 
these cells are not often seen, as they are not numerous, from ten to twenty to each pore, 
when visible their inner ends can be traced into apparent continuity with adjacent 
collencytes. Trichodal protrisenes in dense bundles lie within the walls of the chambers, 
running longitudinally and obliquely, so that their cladomes project into the cavity of the 
chamber, bristling around the pores (PI. XXXIX. fig. 2). 
The Cortex (PI. III. fig. 10). — The thickness of the cortex in full-grown specimens 
varies from about 1*25 to 1’5 mm., the outer part (PI. III. figs. 11, 12) from 0'2 to 
0’4 mm., below the epithelium consists of collenchyma with fusiform cells and granule 
cells strewn through it ; the rest is mainly composed of fusiform cells variously orien- 
tated in planes parallel to the surface. The sigmaspires occur chiefly just below the 
epithelium ; the oxeas crowd the exterior half, so that between the main spicular fibres, 
the lower part, for a thickness of 0’6 mm., is devoid of spicules. 
The Choanosome (PI. III. fig. 13). — The mesoderm is a collenchyma, which is finely 
granular about the flagellated chambers, but of the usual character when it accompanies 
the spicular fibres. The flagellated chambers have an average diameter of about 0*0395 
to 0*043 mm. 
The Young Sponge (PI. III. figs. 2, 3, 9). — In the smallest specimen of the collection 
(under 3 mm. in diameter), the cortex is from 0*08 to 0*16 mm. thick, and the oscular 
tubes and cloacal chambers are not yet completely differentiated. The latter are repre- 
sented by a part of the cortex, which is thinner here than elsewhere (0*016 mm. thick), 
perforated by numerous pores, and curved inwards below the general level of the sponge. 
Thus in their inception the cloacal chambers are simply poriferous areas of the cortex. 
The pores open into sinuses (they can hardly be called canals), left by the folding of the 
choanosome. The oscular tube is represented simply by the margin of the general 
