XXVI 
THE YOYAGE OF H.M.S. CHALLENGEK. 
Canal-System in Placina. — The simplest type of folding producing the simplest type 
of canal-system is that already described by Schulze as occurring in Oscarella lobularis, 
0. Schmidt, and Placina monolopJia, F. E. S. The term folding as used here and b}' 
Schulze is not altogether free from objections, and in many cases the process might be 
better described as evagination or invagination ; thus the folds of Placina may be 
described as vertical tubular outgrowths or evaginations of the spongophare; such 
evaginations remind one of the radial tubes of the Sycones, which Haeckel regards as 
produced by gemmation ; fundamentally evagination and gemmation are much the same 
thing, and the question is chiefly one of terms. 
Canal- System in Sponges loith plate-lihe Walls. — In no other known Tetractinellid 
sponge is such a simple canal-system as the preceding met with ; the nearest approach to 
it — longo intervallo — is made by Epallax callocyathus, a vasiform sponge with a 
lamellar wall. As in most other plate-like sponges, the oscules are distributed in a fairly 
regular manner on the concave side of the plate, the pores in sieves on the convex or 
outer side ; a transverse section reveals two, apparently ectosomal, layers, one forming 
the oscular and the other the poriferous face of the sponge ; between these two layers is 
a regularly folded sponge plate (PI. X. figs. 10, 11); its folds, however, have not the 
simple structure of Placina, but are themselves folded, the flagellated chambers opening 
into the sinuses of the secondary folds ; further, they are only apparently folds, tangential 
sections showing in this case, as in Placina, that the apparent folds are really tubular 
evaginations with a circular lumen. Simple as this structure undoubtedly is, yet in the 
absence of embryological evidence it is by no means an easy task to derive it from the 
original Ehagon. Our first step will be to determine the homology of the two superficial 
layers; that forming the poriferous face is evidently ectosomal, that forming the oscular 
face probably not. What is the nature of the epithelium of the oscular face? Is it 
ectodermal or endodermal ? If endodermal, then the cavity of the vase forming the 
sponge must be paragastral. The vase-like form however is not essential, and in 
other plate-like sponges seldom exists except as a secondarily acquired character ; 
if then we seek to explain the more general case of a flat plate-like sponge on the 
assumption that the epithelium of the oscular face is endodermal, we shall have to imagine 
that it originated from the Ehagon by an opening out of the latter, the oscule of the 
Ehagon expanding till its margin became the margin of the plate, and its paragastral 
surface flattening till it became the oscular face ; this is a possible explanation, and, 
adopting it in an article on Sponges published elsewhere,^ I have called the superficial 
layer of the oscular face an endosome. Another explanation, however, may be advanced. 
Let us regard the epithelium as ectodermal, the superficial layer of the oscular face will 
then consist of an outer ectodermal, an inner endodermal, and a middle mesodermal 
layer, in other words it is not an endosome but a hypophare. We may thus compare 
* Encyclopaedia Britannica, 9th ed., art. Sponges, p. 415. 
