118 
THE VOYAGE OF H.M.S. CHALLENGEE, 
diaphragms ; both vela and collenchyme, however, disappear as the canals subdivide into 
branches. The main incurrent canals are also constricted by vela. 
The cloacal wall is in direct continuation with the ectosome, of which it appears to be 
an invaginated portion, its roof or the oscular margin arising as a prolongation or out- 
growth of the tissue at the angle of flexure. 
The mesoderm of the choanosome is a true sarcenchyma, the separate cells of which are 
sometimes very clearly displayed (PL XII. fig. 42). The flagellated chambers are some- 
what smaller in the specimens from Station 208 than in those from Station 186 ; in the 
latter they are usually about 0’0276 mm. broad by 0‘02 mm. long; in the former they 
vary from 0‘016 to 0'02 mm. in diameter. The aphodal canals are usually short and 
sometimes absent, so that occasionally the chambers are eurypylous. The scleroblasts of 
the large spicules are clearly shown in the thin slices of the sponge, and in some 
instances can be traced extending over the cladome of a triaene, as represented in the 
illustration (PI. XII. fig. 43), which shows an accumulation of scleroblastic protoplasm 
over the cladome of an anatriaene. 
Symbiotic Alga. — The collench 5 niie of the ectosome (PI. XII. fig. 41) and the canal 
walls is infested by an Alga, the presence of which was remarked by Ridley also in the 
specimens he examined. Ridley compares it to a Nostoc, and it appears to closely 
resemble the Phycochromaceous Alga which Schulze observed in Spongelia pallescens, 
and which he named Oscillaria spongelise.^ Both Schulze’s specimens and those in 
Myriastra clavosa. are about 4 mm. in length, but they differ slightly in other dimensions ; 
thus the disc-shaped cells of Oscillaria spongelia are about 0‘006 mm. in diameter and 
0'004 mm. in thickness, while those of the Australian form are 0'008 mm. in diameter 
and 0'0045 mm. thick. 
Myriastra clavosa, var. quadrata, Sollas. 
Myriaster quadrata, Sollas, Prelim. Account, Sci. Proc. Eoy. Dubl. Soc., vol. v. p. 189, 1886. 
Sponge. — Small, free, rounded below, constricted above into three lobes ; on 
the upper surface a depression between the lobes, in which a single small oscule, sur- 
rounded by a membranous margin, is situated. Pores in sieve-like areas, generally 
distributed. 
Spicules. — I. Megascleres. 1. Oxea, straight or curved, fusiform, sharply pointed ; 
2’56 by 0'0158 mm. 
2. Dichotriaene, a straight or curved, conical, sharply pointed rhabdome ; proto- 
cladi projecting outwards and forwards, the deuterocladi extending horizontally. Rhab- 
dome 3‘206 by 0’0276 mm., protocladi O'll mm., deuterocladi 0'27 mm. in length. 
3. Anatriaene, a long, slender, conical, sharply pointed rhabdome, with thin, conical, 
1 Zeitschr. f. wiss. Zool., Bd. xxxii. p. 147, pi. v. fig. 7, pi. viii. figs. 9 and 10. 
