214 
THE VOYAGE OF H.M.S. CHALLENGEK. 
The microstrongyles occupy the position of a somal aster, i.e., they not only form a 
layer beneath the epithelium of the cortex (PL XVIII. figs. 14, 17), but occur dispersed 
throughout the choanosome. They are evidently derived from the large oxyaster, or both 
from a common form like the large aster of Caminus vulcani, to' the microxea of which 
the microstrongyles bear a remarkable resemblance. The tension which led to the modi- 
fication of the sterraster, and the reduction of the oxyaster to a microstrougyle, also 
manifests itself by sometimes, though very rarely, giving a spiral elongation to the 
oxyaster, which then reminds one of the plesiaster of some species of Thenea. 
The chiaster, in some of its varieties indistinguishable from the chiaster of Myriastra 
amongst the Stellettids, first appears in the inner fibrous layer of the cortex, and thence 
extends through the choanosome, chiefly, if not only, occurring beneath the epithelium of 
the canal walls. The oxyaster is confined to the choanosome, and though it also occurs 
along with the microstrongyle and the chiaster immediately beneath the epithelium, its 
more usual position is somewhat more removed from it, deeper within the sarcenchyma. 
Numerous transitional forms of aster occur between the chiaster and oxyaster, so that no 
sharp line of demarcation can be drawn between them. 
Genus 2. Caminus, 0. Schmidt. 
The sterraster is spherical, and the somal microsclere a spherule. The roofs of the 
incurrent chones are cribri-poral ; the oscule is the patent opening of a cloaca. 
Caminus sphseroconia, Sollas (PI. XXVII.). 
Caminus sph^roconia, Sollas, Prelim. Account, Sci. Proc. Eoy. Dubl. Soc., vol. v. p. 196, 
1886. 
Sponge (PI. XXVII. fig. 1). — Massive, growing upwards into massive, rounded 
lobes ; attached. Oscules situated on the summits of the lobes, large, with wide, smooth 
margins, leading into a large cloaca, which receives the excurrent canals, opening over its 
lower half by wide patent mouths. 
Pores in sieves, forming the roofs of incurrent chones ; the upper surface of the 
sponge is raised into a polygonal network of low ridges, which mark out the limits of 
each chone, the polygonal spaces being roofed over by the poriferous membrane. On the 
lower surface the polygonal outlines are less marked. 
The outer layer of the cortex consists of vesicular tissue beneath the epithelium, 
crowded with but one form of microsclere, the spherule. 
Spicules. — I. Megascleres. 1. Strongyle (PL XXVII. fig. 3), small, cylindrical, 
straight, curved, or somewhat rapidly bent in the middle, with rounded ends, 0'5 by 
0 016 mm. 
