222 
THE YOYAGE OF H.M.S. CHALLENGED. 
it is depressed into an annular valley, which surrounds a small mound-like eminence ; the 
poriferous roofs of chones cover the sides of the sponge up to the outer rounded shoulder 
which bounds the valley externally ; they fail over this, but set in again over the bottom 
of the depression, and extend all over]the central mound except at its very summit, where 
they are absent. The cortex is about 1’3 to 1'6 mm. in thickness, the thickness of the 
ectochrote varying from 0'19 to 0‘5 mm. This layer (PI. XXL fig. 10) consists of 
numerous cells lying nearly adjacent to each other, but separated by a thin layer of gela- 
tinous matrix, in which they are embedded ; in their youngest stage they present them- 
selves as oval or oblong masses of granular protoplasm, which stains deeply, so as almost 
to conceal the spherical nucleus and nucleolus which it encloses ; later the protoplasm 
begins to disappear, and first at the margins of the cell, leaving very visible an outer 
cellular membrane, which is quite distinct from the wall of the cavity in the gelatinous 
matrix within which it lies. In many of the cells of this tissue spherasters may be seen 
included. The diameter of the cells is about 0'0197 mm., of the nucleus about 0’0039 
mm. As this tissue approaches the sterrastral layer the included cells lie somewhat 
further apart, and present a more regular oval or rounded form ; the general appearance 
of the tissue is that of a gelatinous matrix with oval cells scattered through it. As it 
approaches the chones it becomes converted into ordinary collenchyma, the collencytes 
of which radiate more or less towards the chonal waUs. The microscleres of this layer 
are all strongylate spherasters (5). 
The sterrastral layer which succeeds is about 1 ’2 mm. in thickness, and presents the 
usual structure. 
The poral roofs are traversed by fusiform cells. The pores lead into short canals 
which unite into long, winding, and branching canals, which extend horizontally through 
the ectochrote. These canals, lined by epithelium which is bulged out by the spherasters 
underlying it, are crossed by velar diaphragms, and each system which they form is con- 
tinued into an incurrent chone. The chones are provided with thick muscular sphincters, 
the myocytes of which are all arranged concentrically (PI. XXL fig. 11), surrounding the 
inner end of the chone. The sphincters lie in the midst of collenchyme, with which they 
stand in sharp contrast (PI. XXL fig. 9). Towards the upper or outer limit of the 
sphincter, fusiform cells terminate abruptly against the lining epithelium, from which 
they extend radially into the surrounding myenchyma. These cells measure about 0‘02 
mm. in length, the nucleus lying about O’Ol mm. from their epithelial ends. Occasionally 
one may also observe darkly stained stellate cells lying in the myenchyma, near the 
inner ends of the fusiform cells. It is possible that we have here a rudimentary 
nervous structure : the fusiform cells may be sensitive, and the stellate cells ganglionic in 
function. As to the fact of the response of the sphincter to a stimulus directly applied 
to it there can be little doubt, since by touching or gently rubbing the sphincter of the 
oscular chones of Pacliymatisma with the point of a pencil, or any other convenient 
