70 
THE VOYAGE OF H.M.S. CHALLENGER. 
The ectosome appears in these as a continuous outer investment, not in the least 
folded, but as even in its curvature as the shell of an egg. The choanosome on the 
contrary is folded within it in a complex manner, so that the excurrent and incurrent 
canals, or rather cavities, are already produced in various orders of size, the smallest in 
communication with the flagellated chambers. The coUenchymatous mesoderm is very 
poorly developed either in choanosome or ectosome ; it is thickest where the folds of the 
former, bulging outwards, become continuous with the latter, producing the earhest 
formed pillars of the subdermal cavities, which are represented at this stage by the 
incurrent cavities or sinuses left between the ectosome and the choanosome by the folding 
inwards of the latter. These cavities also represent the rudiments of the incurrent 
canals, so that we can hardly yet speak of the subdermal cavities as differentiated from 
the incurrent canals. The folding of the choanosome evidently takes place in complete 
independence of the ectosome, and for its accomplishment we must either suppose a 
rapid ingrowth of epithelium from the pores of the latter following the retreating folds of 
the former, or we must suppose a genuine cleavage to occur between the two layers, and 
at present we have no evidence to decide this point. Appearances are in favour of 
cleavage, which, judging from general embryological data, we might regard as a 
substitute for the invagination which we know to occur in other forms of sponges. 
Spicules are already present in the young sponge, anatrisenes were not seen, but 
otherwise all the spicules characteristic of the adult were observed ; of course they are 
very diminutive. 
The species of Thenea are so liable to variation that it is often difficult to find good 
characters for them, or to feel sure in all cases whether one is dealing with a species or a 
variety. I rank Thenea schmidtii as a species because the characters which distinguish 
it are constant throughout a series of specimens, which in other respects, such as external 
form, are very variable, and which were obtained from several different localities. The 
same is true of the other North Atlantic species, Thenea muricata, but it will possibly 
require the examination of many more specimens than have come under my observation 
before the distinction of Thenea schmidtii and Thenea muricata can be regarded as fully 
established. 
The following is a summary of the characters in which they differ : — 
1. Plesiaster. — This spicule is larger and much more abundant in Thenea schmidtii 
than in Thenea muricata. In the latter its numbers sink into insignificance beside 
those of the spirasters, in the former it is as numerous as those spicules or more so, and 
plays quite as important a part in the skeleton of the sponge as the tetrad spicules do in 
Dercitus and its allies. This relative abundance characterises specimens of very different 
size and external form. 
2. Mesoderm. — This is richly developed in Thenea muricata (PI. VII. fig. 3), forming 
a thick wall about all the canals, and converting them by its velar extensions into 
