140 
THE VOYAGE OF H.M.S, CHALLENGER. 
constricted into two divisions by a transverse velum (PL XII, fig. 24). The einthelium 
of the canals is very clearly shown in some parts of the thin slices, and frequently the 
outer membrane of the epithelial cells is supported on the tylote ends of the chiastral 
actines in tent-like projections. The cladi of early forms of the anatrisene project from 
the primitive tylus at right angles to the rhabdome, as shown in figs, 16-18, PI. XII., the 
first two figures representing two successive stages in the growth of the spicule, while the 
third is a more highly magnified representation of the spicule illustrated in fig. 16. The 
cladi of the protrisene project outwards and forwards from the first (fig. 19). The tylus 
from which the cladi originate is well shown in the end view of a young trisene given in 
fig. 22. 
The anthasters show no tendency to constancy in the number of the actines ; six are 
frequently present, but five, four, three, and two are quite as common. The young 
anthaster is smooth, with conical pointed actines (figs. 20, 21) ; it is not an overgrown 
chiaster, but both probably are modifications of a more primitive form. 
Both chiasters and anthasters are found alike in the choanosome and the ectosome, 
but anthasters are more numerous in the latter region than in the former, and are wholly 
absent near the centre of the sponge. The chiasters appear nearer the epithelial surfaces 
than the anthasters ; thus in the ectosome they are plentifully scattered over the inner 
face of the lining epithelium of the subdermal cavities, almost to the exclusion of 
anthasters, which usually are more deeply seated in the coUenchyma. 
Anthastra communis, Sollas (PL XIII. figs. 1-29 ; PL XV. figs. 20-27). 
Anthastra communis, Sollas, Prelim. Account, Sci. Proc. Roy. Dubl. Soc,, vol. v. p. 191, 1886. 
Sponge (PL XIII. figs. 1, 12, 13) nearly spherical when young, spherical, oval, or 
tuberose when older. Free or attached ; frequently sparingly incrusted with foreign 
bodies. Oscules usually not distinguishable from the pores ; in one specimen distinct, 
congregated. Pores in cribriform areas marked out by the cladi of the dichotrisenes. 
Subdermal cavities forming winding, branched, anastomosing canals within the ectosome. 
Excurrent- canals usually small, not distinguishable from those of the incurrent system. 
Surface even, rough to the touch, hispid in places. 
Spicules. — I. Megascleres. 1. Oxea (PL XIII. figs. 2, 14 ; PL XV. fig. 23), fusiform, 
straight or slightly curved, variously pointed, obtusely or sharply, or tornotate or 
strongylate, 4 ’2 to 5*6 by 0’06 to 0’09 mm. A smaller oxea, apparently a young form 
of the preceding, is present, and forms within the ectosome radiating pencils at the 
ends of the radial spicular sheaves. 
2. Dichotrisene (PL XIII. figs. 3, 15, 19; PL XV. fig. 24), a conical rhabdome, 
much attenuated proximally, sharply pointed or rounded off near the end ; cladi bifurcate, 
protocladi projecting forwards and outwards, sometimes more outwards than forwards. 
