REPORT ON THE TETRACTINELLIDA, 
277 
appear to undergo but little change, while the collenchyma is subject to considerable 
modification ; in some places, sometimes near the exterior surface, sometimes in the neigh- 
bourhood of the water-canals, the vesicles which contain the granular cells become enlarged 
at the expense of the intervening collenchyma, which is reduced to forming the thin 
film-like walls of irregular cavernous spaces ; the matrix then ceases to stain with 
hsematoxylin, and the collencytes, which do not lose this property, become all the more 
visible. The granule cells remain within the enlarged vesicles unchanged, appearing now 
dark on the light ground of the field of the microscope, but little interfered with by the 
remaining collenchyma ; Vv^hile in the case first described they appeared as lighter bodies 
on the darker ground of collenchyma. Next to the epithelium of the water-canals the 
granular cells are frequently arranged in a single close layer forming an investing wall 
(PI. VIIL fig. 36). 
This peculiar form of collenchyma is similar to that met with in Pachastrella 
ahyssi and in so many species of Lithistids. 
The ectosome, 0‘4 mm. in thickness, differs from the choanosome chiefly in the 
absence of flagellated chambers, partly in minor details (PI. VIIL fig. 35). The 
collencytes are frequently modified into elongated fusiform cells not more than 0‘02 to 
0’04 mm. long ; running parallel to the surface, sometimes in great numbers, they give 
to the ectosome a fibrous appearance. A few of such fusiform cells may be directed 
at right angles to the surface, their distal ends terminating against the outer epithelium. 
The finely granular cells are as a rule rarer in the ectosome than elsewhere. 
The epithelium of the water-canals is rendered very evident by the deep stain taken 
by the nuclei of its component cells (PI. VIIL fig. 36); by the presence of these little 
almost black dots the epithelium can be traced throughout the canals right up to the 
flagellated chambers, where it becomes replaced by the usual choanocytes. The flagellated 
chambers vary from about 0’02 mm. in diameter to 0’02 by 0'0275 mm.; the incurrent 
canal passes into them by a prosodus, and they communicate with the exeurrent canal by 
a long aphodus ; both prosodus and aphodus are frequently obliterated so far as their 
cavity is concerned, but they can still be easily traced by the line of apposed epithelial 
nuclei where their walls are in contact. 
In the young state the spicules are smooth (PI. VIIL figs. 32-34) and not spined, 
with very slender cladi and rhabdome, the former curved usually more or less sigmately; 
the deuterocladi of the dicho- and tricho-cladose forms are relatively much shorter in the 
young state than subsequently (PI. VIIL figs. 33, 34). They are then suggestively 
similar to the trilophose candelabra of Placina trilopha, F. E. Schulze (p. 279). In 
one or two instances a young spicule was observed within a scleroblast (PL VIIL fig. 38); 
the adult spicule has not been observed in connection with a scleroblast, but nuclei similar 
to those of the collencytes are sometimes to be met with, forming a series down its 
sides (PI. VIIL fig. 37). 
