xlii 
THE VOYAGE OF H.M.S. CHALLENGER. 
main water-canals and parallel to the surface in the cortex. Transverse strands also 
extend through the cortex, the most marked being those which accompany the radiating 
spicular fibres. Owing to their generally tangential direction in the cortex a transverse 
section presents the appearance shown in PI. XVI. fig. 13, ^.e., tracts of fibres are seen cut 
across transversely, surrounded by others cut longitudinally. In the transverse sections 
the hyaline wall appears as a round or oval area, enclosing a circular, granular spot 
in the centre, which represents the axial fibre cut across. CoUencytes are almost always 
associated with the inocytes of the fibrous strands, and sometimes granular- or pigment- 
cells as well. In many sponges {Pilochrota gigas, Psammastra murrayi, Tethya 
lyncurium, and many others) strong bands are given off from the cortex, sometimes as 
much as 20 mm. in length, for attachment to stones, shells, or other foreign objects. 
These (PI. XX. figs. 9-13) chiefly consist of longitudinal strands of inocytes or 
myocytes. 
Myocytes. — These are long, granular, fusiform cells, enclosing a small oval nucleus 
with a spherical nucleolus in the middle. A thin layer of hyaline material is frequently 
present around them, or they may occur simply immersed in a homogeneous, gelatinous 
matrix. They differ from inocytes chiefly in the reduction or suppression of the hyaline 
sheath of the latter, and in the constantly granular character of the axial thread, which 
in the myocyte forms the chief part of the cell. CoUencytes are associated with the 
myocytes and can sometimes be traced into continuity with them (PI. XXXIX. fig. 10); 
either the filamentous end of the protoplasmic portion of the myocyte, or small fibrils 
extending from its sides in the neighbourhood of the nucleus, passing into the processes 
of a branching collencyte. 
The myocytes chiefly occur concentrically arranged about the openings of the water- 
canals ; thus they are always present about the central aperture of the vela and form a 
thick sphincter representing an enlarged and highly muscular velum about the ends of 
the chones. Radiately arranged myocytes usually are associated with concentric ones 
in a velum, crossing the latter transversely. In the case of a typical chone the inner 
end lies in the inner (inocytal) layer of the cortex, and the sphincter is evidently only 
a modified portion of this layer, into which it gradually passes. The myocytal ring 
or muscular sphincter is, however, always easily distinguished, even when examined 
in unstained preparations and under a low magnification. This is due to the extreme 
thinness of the hyaline sheath about the axial protoplasm, so that the protoplasmic 
bodies of the myocytes lie almost in contact with each other, and thus produce a darkly 
granular ring of tissue, which is in marked contrast with the clearness of the inocytal 
layer, in which the protoplasmic threads are separated by the thickness of two 
comparatively thick hyaline walls. The radiating myocytes of a velum are frequently 
attached at one end to the epithelium lining the margin of the central aperture ; in 
such a case the point of attachment may frequently be observed sunk below the general 
