XXll 
THE VOYAGE OF H.M.S. CHALLENGER. 
In the simplest sponges with an aphodal chamber-system, such as Myriastra (PL XII. 
figs. 24-27) among the Stellettidse, the ectosome does not markedly differ from that already 
described, but in genera only slightly removed from this, such as Pilochrota, it exhibits 
in different species a surprising diversity of structural detail. The first advance appears 
to consist in a growth of the mesoderm, leading to a thickening of the pillars of the sub- 
dermal cavities, and to the extension inwards of processes from the dermal membrane. 
In this way the greater part of the subdermal cavity may become obliterated, special 
interspaces or canals being left, through which the pores communicate with the incurrent 
canals. The growth of the mesoderm is accompanied by structural differentiation ; 
additional fusiform cells make their appearance, especially in the lower layer of the 
ectosome, i.e., that next the choanosome, and these increase in number till they form 
dense fibrous bundles, which cross each other in all directions, chiefly parallel to the 
surface, and so produce a thick fibrous felt. The ectosome is thus differentiated into 
two regions or layers, an outer chiefly collenchymatous, and an inner chiefly fibrous. 
As in the dermal membrane of Tetilla pedifera, a thin layer of fusiform cells, 
probably myocytes, is present immediately beneath the outer epithelium of the surface 
of the sponge. 
In this and further stages of development the ectosome is known as a cortex. 
In some species of Pilochrota [Pilochrota pachydermata, Pilochrota gigas), the 
development of fusiform cells continues till the cortex becomes entirely fibrous through- 
out (PL XXXYIII. fig. 25). Striking as is the difference in general appearance of a 
wholly fibrous cortex and one in which the outer moiety consists chiefly of collenchyma, it 
is a character of no special significance, at all events in classification ; thus in the Monaxon 
genus Tcthya, closely allied, almost identical, species may present, the one a cortex 
consisting of an outer collenchymatous and inner fibrous layer [Tcthya seychellensis, 
PL XLIV. fig. 4), and the other a cortex fibrous throughout [Tcthya ingalli, PL XLIV. 
fig. 16). This difference in the cortex is not, however, without its effect on the character 
of the intercortical cavities ; in the last named or entirely fibrous cortex these cavities 
are usually simple cylindrical tubes extending directly across the cortex from the pores or 
pore-sieves to the incurrent canals of the choanosome (PL XLIV. fig. 14) ; in cortices with 
an outer collenchymatous layer, on the other hand, the cavities remain simply tubular 
within the fibrous region only, but within the collenchyma extend laterally parallel to 
the surface, burrowing through it as branching canals (PL XL. fig. 8), or widely 
excavating it as continuous chambers (PL XL. fig. 3). 
In sponges with a well-developed cortex, the intercortical canals are usually of a 
very definite character (PL XL. figs. 3, 8 ; PL XXL figs. 9, 29), and at their inner end, 
where they communicate with or pass into the incurrent canals, they are usually pro- 
vided with a muscular sphincter, which represents an over-developed velum. They 
have been called “ chones,” and distinguished into an outer part which extends from the 
