62 
THE VOYAGE OF H.M.S. CHALLENGER. 
essential respect similar to the oxytylote so characteristic of Esperia marshal-halli 
(S. Kent). 
Ectosome (PI. VIII. fig, 9). — This may he regarded as that outer layer of tissue in 
which the subdermal cavities are included. On an average it measures 0'3 mm. in thick- 
ness. The subdermal cavities occupy the greater part of it ; above them it is reduced 
to a dermal membrane 0’004 to O'OOS mm. thick; between them it forms pdlars 
traversed by the megascleres, and connecting the dermal membrane with the 
qhoanosome. The collenchyma of the ectosome is of the normal type; and so well 
preserved that the union of the collencytes by the anastomosis of their branching 
processes into a continuous network is particularly well displayed. Some of the 
processes of the collencytes can be traced up to the epithelium, in which they appear 
to terminate, — in what way I could not precisely ascertain, but it appeared to be by 
coalescence with the epithelial cells. 
The ectosome contains generally but few myocytes, which lie immediately below the 
outer epithelium tangential to the surface, and in the pillars of the subdermal cavities 
where they are radiately or longitudinally directed ; but where it becomes modified to 
form the poriferous membrane of the equatorial recess, and the membranous margin of 
the oscule, myocytes constitute the greater part of it, so that in these regions the whole 
of the middle layer between the outer and inner epithelia may be regarded as a muscular 
sheet. 
In the wall of the cloaca just below the oscular margin (PI, VI. fig. 18), the 
collenchyma becomes modified by the abundant development in it of certain elements 
which elsewhere are only sparingly present (PL VI. fig. 19); these are more or less oval 
cells with very sharply defined outlines, from 0‘02 to 0*04 mm. in diameter, enclosiog 
one, two, or several more or less spherical vacuoles, in at least one of which is usually 
situated a deeply stained transparent globule, 0*004 to 0*008 mm. in diameter; probably 
of the same nature as similar globules described in my paper on Thenea wallichii} The 
granular protoplasm of the cell forms a network of w^hich the vacuoles are the meshes, 
and in one of its widest trabeculae, from which the others radiate, is situated a nucleus, 
0*012 mm. wide, enclosing a spherical nucleolus, 0*004 mm. in diameter, which is attached 
to the sides of the nucleus by radiating threads. The exterior of the cell is produced 
into slender branching processes like those of a collencyte, and it may possibly be regarded 
as a collencyte modified to form a reserve of nutriment (thesocyte). 
Choano&ome (PI. VI. fig. 17 ; PL VIII. fig. 9). — The collenchyma of the choanosome 
is reduced to a minimum, being present as the merest film between adjacent flagellated 
chambers, and as a very thin layer forming the walls of the larger water-canals, which 
are without vela, and scarcely modified from their primitive character as spaces left by 
the folding of the sponge-wall ; collenchyma also sparingly accompanies the spicular tracts. 
1 Ann. and Mag. Nat. Hist, vol. ix. p. 447, pi. xyII. figs. 18, 19, 43-46, 1882. 
