88 
THE VOYAGE OF H.M.S. CHALLENGER. 
Remarks. — Several individuals of this species occur embedded in the specimen of 
Calthropella geodiides, Carter, described on page 110. The largest, which bears three 
oscules, is 37 mm. long, 25 mm. broad, and 10 mm. high. The oscules, usually oval, 
measure about 9 by 6 mm., the depth of the surrounding fringe of oxeate spicules is 
7 mm. and over. The fringe is sometimes erect, but more often very obliquely inclined, 
sloping over the oscule on one side, and away from it on the other. The margins 
of the oscule are produced over its base into a short tube, an extension of the cloaca. 
The cribriform membrane of the cloaca is attached below to the maro’ins of the 
O 
terminal apertures of the excurrent canals. Along the line of attachment it is not 
perforate, and this imperforate tissue forms a kind of framework enclosing oval sieve-like 
areas, which cover the ends of the excurrent canals. The fenestrse of the sieves vary from 
0'16 to 0*024 by 0*34 mm. in diameter. 
The ectosome is vaguely defined from the choanosome ; and subdermal cavities are not 
present. 
The mesoderm is a collenchyme of the ordinary type ; it forms thick walls about the 
chief canals, which are frequently constricted by vela, and so converted into a succession 
of vesicles. The flagellated chambers open by wide mouths into the ultimate excurrent 
canals which are not thickly lined by collenchyme ; indeed, the regions occupied by the 
chambers are remarkably poor in this tissue, the chambers lying closely contiguous, so 
that the existence of the mesoderm between them is scarcely ap23arent, and best indicated 
by the metasters which accompany it. The chambers measure from 0*032 by 0*036 mm. 
to 0*036 by 0*051 mm. 
Skeleton. — The chief constituents are microxeas, which, from dimensions no greater 
than usual, increase to such a size that it is difficult to delimit them from the immature 
megascleral oxeas. They thus present a striking instance of the passage of a microsclere 
into a megasclere. They are very irregularly distributed, forming a loose felt. On the 
whole, there is a tendency to take up a position tangential to the walls of the canals. 
The size of the spicule seems to depend to a great extent on its position ; it is checked 
by meeting the opposition of a free surface — thus, when it lies parallel to the axis of a 
canal it may attain a considerable length, but when it runs transversely to two epithelial 
surfaces facing each other, it will not exceed the distance between them in length. 
The spirasters are most abundant in the ectosome, beneath the outer epithelium, but 
they also occur beneath the epithelium of the main canals, where these are furthest 
removed from the exterior. The metasters are most abundant in the choanosome, though 
they are sparingly present beneath the outer epithelium. The oxeas are distributed quite 
irregularly within the sponge, only becoming radiate in direction close to the surface ; as 
already mentioned, around the oscule they project far beyond the surface. The calthrops 
are irregularly distributed also, but the orthotrisenes lie radiately, their cladomes 
tangential to the surface, and the rhabdomes at right angles to it. The ‘‘ sausage 
