Ixxxvi 
THE YOYAGE OF H.M.S. CHALLENGEE. 
plane as that of the aperture, in these instances the oscular membrane is also extended in 
the plane of the oscule. In Myriastra, another of the Stellettidse, the openings of the 
excurrent canals within the cloaca are sometimes hispidated by small oxeas (p. 117). 
Spicules of the Incurrent Openings. — The margins of the pores, whether occurring on 
the outer surface of the sponge or in special vestibular recesses {Cinachyra) are in the 
Tetillidse frequently hispidated by trichodal spicules, usually protrisenes ; the excurrent 
pores in the cloacas of Cinachyra are similarly hispidated. 
In the genus Thenea the margins of the equatorial poriferous recesses are fringed with 
long spicules, which project outwards and downwards where they fringe the upper margin, 
and upwards and outwards from the lower margin ; when the recesses are circumscribed, 
forming several oval areas, the spicular fringe becomes a tube ; occasionally the upper 
fringe is united to the lower by a strong thread of fibrous tissue. 
Modifications of the Radiate Type. — In some sponges, chiefly the Tetillidse, the radial 
fibres are spirally twisted about one diameter of the sponge, usually the vertical ; there is 
no constancy in the direction of the twist, the spiral being as often left as right handed 
{ride p. 25). This modification may be explained as resulting from a difference in the 
rate of growth of the choanosome or spicular tracts and the cortex. They would appear 
to be spiral curves of pressure. 
Many sponges which possess a radiately arranged skeleton when young lose all traces 
of it or any other arrangement when they attain to larger growth ; this is the case with 
Pachymatisma and probably with most sponges which are closely related to species 
with a radiate skeleton, but which are without it themselves ; as instances may be cited 
Caminus, Erylus, and many others. 
Cortical Spicules. — With the differentiation of the cortex there arises the possibility 
of a further differentiation of spicules ; in the Tetillidse cortical spicules occur in most 
corticate genera, in Craniella they are confined to the inner and fibrous layer of the 
cortex, which they traverse not quite radially but with an inclination a little on each side 
of a true radial; in Cinojchyra they are more nearly radial in direction, but occur chiefly 
in the outer three-quarters of the cortex, which, however, is fibrous throughout ; in 
Chrotella they lie chiefly tangentially in the cortex, but without any precise arrange- 
ment. In none of the remaining Choristida do cortical megascleres play so important a 
part as in the Tetillidae, occasionally in the Stellettidse and the Geodiidse small cortical 
oxeas are present hispidating the outer surface of the sponge, though without contri- 
buting largely to its support as they do in the Tetillidse. In the Geodiidse, however, 
hispidating cortical spicules may sometimes be observed, which though of apparently 
slight importance in the economy of the sponge itself, are of great interest owing to the 
resemblance which they bear to certain small cladoxeas described by Eidley and Dendy 
as hispidating the cortex of a Suberite {Proteleia sollasi) these are very minute 
1 Eidley and Dendy, Ann. and Mag. Nat. Hist, ser. 5, vol. xviii. p. 152, pi. v. 
