KEPOKT ON THE TETKACTINELLIDA. xxiii 
sphincter to the investing epithelium of the sponge (ectochone), and an inner which 
extends from the sphincter to inner limits of the cortex (endochone). The endochone is 
more frequently absent than present ; its existence depends on the position of the chonal 
sphincter ; if this lie in the same plane as the inner limit of the cortex there will of course 
be no endochone. I think, therefore, the distinction between ecto- and endo-chone may 
as well be suppressed, the existence of an endochone when present being expressed by 
the statement that the chonal sphincter is situated at such and such a level within the 
fibrous layer of the cortex. In some cases, however, the terms ectochone and endochone 
will be met with in the descriptive part of the Eeport. 
Beneath the cortex and lying in the choanosome there occur in many sponges large 
lacunar cavities, into which one or more, usually more, of the chones open, and from 
which the incurrent canals take their origin. These cavities are the subcortical crypts. 
The homology of the chones and subcortical crypts has not yet been investigated ; 
Vosmaer^ has thrown doubts on the equivalence of the chones of different families, and 
Marshall^ has suggested that the subcortical crypts are equivalent to subdermal cavities. 
The subject is a difficult one, but in two cases, which are probably typical, the evidence 
seems fairly clear. 
Type 1. Stelletta phrissens . — In very young examples of this sponge the choano- 
some is to be seen distinctly folded within a thin investing ectosome or dermal layer, 
no more advanced in character than say that of 
Tetilla pedifera ; the outer sinuses of the choano- 
somal folds represent both subdermal cavities and 
incurrent canals, for these are not at this stage 
differentiated from each other. In more advanced 
specimens the ectosome, still bearing the same 
relation to the incurrent sinuses and the choano- 
some, is converted into a comparatively thick 
cortex which consists chiefly of collenchyma, faced 
on its inner surface by a layer of fibrous tissue 
(PI. XVI. fig. 20). Large cylindrical canals 
traverse this cortex and give rise to the chones 
of the adult sponge ; while the subcortical crypts 
arise from the incurrent sinuses, and are clearly 
homologous with the subdermal cavities of Tetilla 
and other sponges. The chones on the contrary 
are clearly of independent origin, secondary cavities formed within a cortex, which may 
be regarded as produced by a centrifugal growth of a simple dermal membrane. At 
Fig. VI.' — Longitudinal median section of a young 
Stelletta phrissens, showing the choanosome folded 
within the cortex ; o, oscule. 
^ Bronn’s Klassen u. Ordnung. d. Tliierreichs, Porifera, p. 126, 1887. 
2 Zool. Jahresber. Neapel, p. 186, 1880. 
