CXXIV 
THE VOYAGE OF H.M.S. CHALLENGER. 
constituting one-half the thickness of the whole cortex. These two layers may he 
distinguished as the inner and outer layers of the cortex. In the Sponges with a cortex 
excavated by intercortical cavities, the pores are distributed generally in sieve-hke 
groups over the whole external surface ; in Cinachyra they are restricted to special 
poriferous recesses, invaginated into the choanosome, hence in this sponge there is no 
longer any need for intercortical cavities, and accordingly the cortex is solid throughout, 
consisting mainly of a densely fibrous layer of fusiform cells, with a minimum of 
associated gelatinous matrix. 
The Oscules may be the simple openings of single excurrent canals (Tetilla), or the 
mouths of special cloacal recesses {Chrotella macellata, Cinachyra barhata), or each may 
be the mouth of a system of intercortical cavities, which receive several excurrent canals. 
These canals pass through the inner layer of the cortex and open into the intercortical 
cavities by sphinctrate apertures {Craniella). 
Associated with the progressive modification of the ectosome are certain changes in 
the character of the mesoderm and the chamber-system ; the collenchymatous mesoderm 
of the non-corticate species {Tetilla) becomes a granular collenchyma in the simpler 
corticate species {Chrotella), and a true sarcenchyme in the higher {Craniella) ; corre- 
spondingly the flagellated chambers pass from a eurypylous to an aphodal stage, at the 
same time becoming reduced in size (from about 0'071 by 0'044 mm. in Tetilla, to 0'025 
by 0’035 mm. in Craniella). 
Genital Products. — Ova have been met with in Chrotella macellata, and spermatozoa 
in sperm-clusters in several species, in Tetilla pedifera, Tetilla grandis, Chrotella 
macellata (in which they occur in vast numbers in the vicinity of the cloacas), and in 
Craniella schmidtii ; they have also been observed in Craniella cranium. 
Development. — The embryos and young Sponges observed in Craniella simillima and 
Craniella schmidtii are described on pp. 33, 40. 
Genus 1. Tetilla, 0. Schmidt. 
Tetilla, 0. Schmidt, Spong. Ktiste v. Algier., p. 40, 1868. 
„ O. Schmidt, Spong. Atlant. Gehiet., p. 66, 1870. 
„ Sollas, Sci. Proc. Eoy. Dubl. Soc., vol. v. p. 179, 1886. 
The ectosome is never differentiated to form a cortex, and never provided with 
special spicules. It may be absent altogether, i.e., represented only by the layer of 
investing epithelium, or it may exist as a layer of soft fibro- vesicular collenchyma, 
as much as 0'5 mm. thick. The mesoderm is a collenchyma ; and the canal-system 
eurypylous. The megascleres are arranged in radiating fibres, but separate oxeas are 
also present, crossing those of the fibres transversely. 
Type — Tetilla euplocamus, 0. Schmidt. 
