114 
THE VOYAGE OE H.M.S. CHALLENGEK. 
frequently collected together into clusters, and though most conspicuous in the ectosome, 
are not confined to it, but extend generally throughout the sponge. 
The subdermal cavities communicate with the exterior by short, dome-like, upward 
extensions, which, penetrating the roof, open in the pores. The sarcenchyme of the 
choanosome extends up to the epithelium of most of the canals ; but the main excurrent 
canals are surrounded by collenchyme, which forms a strong partition between them, 
where they approach each other about the axis of the sponge. The collenchyme in this 
position is rendered highly fibrous by numerous fibrillated fusiform cells irregularly 
distributed through it. The main incurrent canals are crossed by vela. 
The flagellated chambers communicate with the excurrent canals by short wide 
aphodi, which are sometimes absent, the chambers then becoming eurypylous. 
A complete passage can be traced from the young trisenes with simple cladi, which 
lie in the choanosome, up to the adult forms with bifurcate cladi, which extend beneath 
the outer epithelium. The chiasters are most numerous in the ectosome, and in the 
walls of the larger excurrent canals ; in the neighbourhood of the flagellated chambers 
they are rare. 
Myriastra dwiplicifurca, Sollas (PI. XII. figs. 29-33). 
Myriaster simplidfurca, Sollas, Prelim. Account, Sci. Proc. Eoy. Dubl. Soc., vol. v. p. 189, 1886. 
Sponge (PI. XII. fig. 29). — Small, rounded, compressed, cylindrical, produced into 
lobes below ; one small oscule on the upper surface,^ pores in sieves generally distributed. 
Spicules. — I. Megascleres. 1. Oxea (PI. XII. fig. 30), fusiform, straight or curved 
slightly, usually sharply pointed, 2’0 by 0’0316 mm. 
2. Orthotrisene (PI. XII. fig. 31), a straight conical rhabdome attenuated to an 
exceedingly sharp point, and with three simple cladi springing forwards at an angle of 
about 45° with the produced axis of the rhabdome, and then extending horizontally ; 
rhabdome 2'325 by 0‘055 mm., cladi 0‘366 by 0‘054. 
3. Anatrisene (PI. XII. fig. 32), a straight, slender, cylindrical, usually very sharply 
pointed rhabdome, and curved, conical, sharply pointed cladi, which proceed from the 
rhabdome almost at right angles for about half their length, and then curve backwards 
till nearly parallel with it. The front of the cladome is thus usually^ flattened, though 
sometimes rounded; rhabdome 1‘86 by 0'0296 mm., cladi 0T2 mm. long, chord 
0‘127 mm. 
II. Microscleres. 4. Chiaster (PI. XII. fig. 33), actines tylote, 0'012 mm. in 
diameter. 
Colour. — Yellowish- white. 
Habitat. — Station 186, September 8, 1874; lat. 10° 30' S., long. 142° 18' E. ; 
depth, 8 fathoms ; bottom, coral mud. 
1 The sponge as represented in the plate is inverted. 
