REPORT ON THE TETRACTINELLIDA. 
425 
O’l mm. on an average ; over the entrance to the incurrent tubes it is perforated by a 
single wide oval aperture, over the inner end of which an exceedingly thin cribriform 
membrane extends — the poriferous roof. 
The endosome or hypophare of the excurrent face is similar to the ectosome, except 
that no poriferous roof extends over the openings of the excurrent camds ; at the same 
time these canals do not each open by a single aperture, but by several perforations which 
traverse the endosome immediately over them. The excurrent canals expand towards 
their termination, so as to produce cavities within the endosome corresponding with the 
subdermal cavities of the incurrent face. 
The ectosome is densely crowded with calthrops-like asters, and faced wdth a dense 
layer of smaller asters. Both kinds of spicules are richly dispersed through the 
choanosome, the smaller asters chiefly occurring below the epithelial surfaces, and the 
calthrops between the flagellated chambers, to which they form a spicular framework 
(PL X. fig. 11). 
Punning through the middle of the sponge-wall, and destroying the regular disposition 
of the canals where they occur, are longitudinal bundles of the stout oxeas, which are 
irregularly coated with spongin ; the spongin appears to be most developed where the 
spicules lie in closest contact; in one instance the remains of sponginoblasts were ob- 
served coating the spongin. 
The mesoderm consists of collenchyma, which for the most part is present as a very 
thin layer, owing to the abundant development of the flagellated chambers ; in places, 
however, it acquires a greater thickness. It contains not only the usual collencytes, but 
granular cells like those described m. Pcecillastra schulzii (p. 81, PL IX. fig. 25), here how- 
ever not so numerous. They are about 0'02 mm. in diameter, the large oval nucleus 
measures 0’016 by O'OllS mm. in diameter, and the spherical nucleolus is O'OOS mm. in 
diameter. 
The axial oxea is subject to considerable modification of form, it frequently terminates 
prematurely in rounded ends, and sometimes, though rarely, becomes tylote at one 
extremity. Occasionally it passes into a globular form, and globules thus formed by the 
reduction of the oxea sometimes occur united several together into a single mass (PL X. 
figs. 7-9). 
Family II. Doryplerid^. 
Centrospinthara in which the ectosome is not a cortex, and the choanosome is not 
regularly folded ; the mesoderm is collenchymatous. The megascleres are oxeas arranged 
without order. The microsclere is a large oxyaster. 
(ZOOL. CHALL. EXP. — PART LXIII. — 1888 .) 
Err 54 
