152 
THE VOYAGE OF H.M.S. CHALLENGER, 
invested by the outer epithelium, below which, as in the adult sponge, is a single layer 
of minute spherasters. Beneath the ectosome lie the subdermal cavities (PL XVI. 
fig. 15), about 0’161 mm. high, measured radially, and of very various widths. The 
pillars between the adjacent cavities are in many cases devoid of flagellated chambers, 
in others the chambers extend more than halfway into them ; the boundary between 
the ectosome and the choanosome is therefore inconstant and ill defined. All the 
forms of spicules which the sponge will at any time possess are already developed, 
but the oxyasters are very rare, and are evidently later developments than the 
spherasters ; the other spicules also, except the spherasters, are very much smaller than in 
the adult, and of quite difierent proportions. The dichotrisenes, the cladomes of which 
already support the ectosome, have thus a very different appearance from that which 
they present later, but almost precisely resemble the young dichotrisene of Thenea. The 
smallest measured presented a rhabdome 0'039 mm., protocladi O'OIS mm., and deutero- 
cladi 0'045 mm. in length ; thus the deuterocladi alone are longer than the rhabdome, 
and an entire dichocladus is almost twice as long. In the fully grown sponge, on the 
other hand, the rhabdome is about seven times the length of the dichocladus ; in other 
words, while the dichocladus increases in length about seven times, the rhabdome increases 
almost ninety times in passing from the youug to the adult state ; at the same time the 
protocladi increase in lengtl^ relatively to the deuterocladi ; in the young sponge the 
latter are about three times the length of the former, in the adult not more than twice. 
In this stage the young sponge forcibly reminds one of a Thenea, and from a young 
Thenea only essentially differs in the characters of the asters, and in the presence of a 
sarcenchymatous mesoderm. 
The young sponge (PI. XVI. fig. 17)^ next to be described was very fortunately 
orientated with regard to the plane of the razor, and, owing to this, reveals in the clearest 
manner the relation of the chones and subdermal cavities to each other, as well as the 
nature of the canal system generally. It was considerably larger than the preceding, 
about IT by 1‘27 mm., but still approximately spherical. The ectosome forms a con- 
tinuous layer of unequal thickness, about OT mm. on an average, entirely surrounding 
the sponge ; within it the choanosome is seen as a strongly folded layer, connected with 
the ectosome only where the outer crests of the folds become continuous with it. The 
outside sinuses of the folds are the incurrent canals not yet differentiated from the 
subdermal eavities, which are now merely the broad outer ends of the outer sinuses 
bounded by the ectosome and by the pillars through which this and the choanosome pass 
into each other. The inside sinuses of the folds are the excurrent canals, and the common 
cavity with which they are continuous is the remains of the paragaster, which communi- 
cates with the exterior by a single oscule. 
^ A woodcut in which the structure represented in this figure is worked out in greater detail is given in the 
introduction. 
