XXIV 
THE VOYAGE OF H.M.S. CHALLENGER. 
the most they can only be regarded as centrifugal extensions of the subdermal 
cavities. 
To this type the cortex of the Geodiidse may be assigned, as will appear from 
observations on a young Geodine sponge described later. 
Type 2. Craniella simillima. — An altogether different history is presented by the 
cortex of this sponge. In the adult, however, the cortex appears to be exactly homologous 
with that of Stelletta flirissens. It consists (PI. II. figs. 12, 13) of (1) a thick fibrous 
inner layer, crossed by cylindrical canals, sphiuctrate at their inner ends, and (2) a thick 
outer collenchymatous layer widely excavated by extensive cavities, which are large and 
continuous in the young sponge, but in older examples become subdivided by a rich 
development of trabeculae, which increase in thickness till they reduce the originally 
simple cavity to a collection of lacunar spaces. The first-mentioned layer (1) would 
naturally be regarded as equivalent to the inner fibrous layer with that portion of the 
chones traversing it in the cortex of Stelletta; the second (2) to the outer collenchymatous 
Fig. VII. — The two types of cortex and their associated canals. A. Stellettid type. B. type of Craniella. sc., subcortical 
crypt ; homologous with s.d., subdermal cavity ; col., collenchymatous layer of the cortex ; fb., fibrous layer ; 
ij/, choanosome ; ch., chone ; p., pore ; sp., sphincter. 
layer of that cortex and the outer ends of several chones run together. Such, however, 
is not the case. 
In the youngest observed specimens of Craniella simillima^ taken out of the parent 
and measuring 1‘4 by 2’4 mm. in length and breadth, the cortex is subdivided into two 
regions, one of which, partly fibrous and much the thicker, is continuous with the choano- 
some, from which it evidently arises by metamorphosis ; the other forms a thin investing 
membrane separated from the inner thicker layer by wide and deep cavities, which are 
evidently the widely extended superficial ends of the incurrent sinuses or true subdermal 
cavities (PI. II. fig. 19). In this case the cortex is evidently of a composite nature, 
consisting partly of the dermal membrane, partly of the metamorphosed ends of the 
excurrent lobes of the choanosomal folds ; in Stelletta, on the other hand, the cortex is 
derived entirely from the dermal membrane by a thickening and differentiation of its 
mesoderm. The chones consequently are secondary formations, and the intercortical 
cavities of Craniella are not homologous with them but with the subcortical crypts. 
