REPORT ON THE TETRACTINELLIDA. 
153 
The mesoderm of the choanosome is a sarcenchyma in this as in the preceding 
individual, and in both the flagellated chambers are aphodal. The aphodal canals have 
not the appearance of metamorphosed folds, and are more probably produced by the 
growth of the surrounding mesoderm converting the original eurypyles into narrow 
canals. 
The ectosome evidently does not share in the folding of the choanosome; the two 
regions appear to behave as independent structures. We may imagine their separation 
to be produced in one of two ways : either an invagination of ectoderm may occur at 
various points on the surface of the sponge and there initiate the inwardly directed folds, 
or an actual cleavage may occur along the lines now occupied by the subdermal cavities. 
The ectosome in this sponge consists of collenchyma, excavated by a few incon- 
spicuous vesicles, which, on close examination, are found to communicate both with the 
exterior and the subdermal cavities. 
The third young sponge examined (PI. XVI. fig. 19) measured about 2'0 by 2*6 mm. 
The subdermal cavities are now well marked off from the incurrent canals, owing to the 
continued folding of the choanosome ; they form an almost continuous cavity about 0‘214 
to 0'36 mm. thick beneath the ectosome, which is twice as thick as in the preceding 
stage (0‘214 mm.). It consists of collenchyma containing fibre-cells, partly more or less 
irregularly dispersed, partly forming a thin layer immediately beneath the subepithelial 
spherasters, and partly arranged as a thin felt over the inner face of the ectosome. This 
last-named felted layer is the rudiment of the thick fibrous part of the adult cortex. 
The vesicular cavities which appear in the ectosome in the preceding stage have now 
acquired a comparatively large size, and present a striking feature in the young cortex 
(PI. XYI. fig. 20). They extend completely across it, and are evidently destined to 
become the chones of the adult. The cortex is thus formed by the growth and differen- 
tiation of a dermal membrane or ectosome, the subcortical cavities are the remains of 
subdermal cavities, and the chones independent structures, not produced by the original 
folding of the sponge, but arising subsequently, in connection with the thickening and 
differentiation of the ectosome. 
Subfamily 3. Sanidastertna. 
Heterasterose Stellettidse, in which the additional microsclere is a sanidaster. 
Genus 6. Tribrachium, Weltner. 
Sponge produced into a special cloacal tube, the megascleres of which are orthodisenes. 
The microscleres are sanidasters. 
(ZOOL. CHALL. EXP. — PART LXIII. — 1887.) 
Err 20 
