156 
THE VOYAGE OF H.M.S. CHALLENGER. 
fusiform cells, different in character to the others, extend from the collenchyma radially 
across the outer fibrous layer to end against the investing epithelium. 
Choanosome . — The mesoderm, except where it forms the walls of the larger excurrent 
canals, is a sarcenchyma ; about the large canals it is a very typical collenchyma, pre- 
senting a colourless unstained matrix and numerous collencytes with long branching 
processes ; it also contains numerous fusiform cells, which are evidently modified collen- 
cytes. In the velar diaphragms of the larger excurrent canals one finds displayed in the 
clearest manner the following structure : — Immediately around the central perforation of 
the diaphragm is a narrow ring of concentrically arranged fusiform cells, which we may 
regard as myocytes ; surrounding this follows a layer of collenchyma containing numerous 
fusiform cells, all radiately disposed, and again in some instances surrounding this a thick 
layer of collenchyma with concentrically arranged fibres. The layer of radiating cells 
varies in thickness; it is frequently 0'04 mm. across. The cells themselves are very 
similar in character to myocytes, a slightly difierent appearance being sometimes pro- 
duced through the abbreviation or supj)ression of the outwardly directed filamentous 
extremity, and sometimes by the conspicuous appearance of the oval vesicular nucleus 
which bulges out the sides of the cell. In one direction, they end against the epithehum 
of the velar aperture ; in the opposite direction, away from the aperture, they terminate 
in one or more fibrils which lose themselves amidst the surrounding collenchyma, with 
the collencytes of which they appear to become continuous. Along with the radiating 
fusiform cells collencytes are associated with more or less regularly arranged branching 
processes ; thus one process usually runs radiately up to the velar epithelium, two others 
given off from opposite sides take a concentric direction, and others again are given off 
from the inner face and extend more or less radiately into the deeper lying collenchyma. 
This contains a plexus of cohencytes and a number of more or less fusiform cells on the 
whole concentrically arranged. 
Ova . — The sarcenchyma contains numerous round or oval cells, consisting of deeply 
stained granular protoplasm, but without yelk granules, enclosing a spherical nucleus 
with a spherical deeply stained nucleolus. These cells lie in cavities lined by so-called 
endothelium ; they are of various sizes, the largest seen measured 0'056 mm. in diameter, 
its nucleus 0'0158 mm., and nucleolus 0'005 mm. in diameter. They are probably ova. 
Canal- System . — The pores, 0‘025 mm. to 0‘06 mm. in diameter on an average, 
though exceptionally larger, lead into irregular intercortical canals, which communicate 
through the inner cortical layer with the incurrent canals of the choanosome. These run 
more or less radially towards the centre of the sponge, interdigitating with the radial 
branches of the excurrent canals, which originate within the choanosome in circular or 
oval apertures in the immersed part of the cloacal tube. Thence they extend root-like 
into the choanosome, running somewhat concentrically within it, but giving off branches 
which, with their subdivisions, run nearly radially towards the cortex. The fiagellated 
