REPORT ON THE MONAXONIDA. 



XXX 111 



placed, outwardly-projecting discastra (PI. LI. fig. ih, d). Beneath this comes a much 

 thicker layer of very peculiar, vesicular tissue. This is composed of closely packed, 

 perfectly transparent, globular cells (PI. LI. fig. Ih, v.c.) about 0"018 mm. in diameter, 

 and each with a small, round nucleus excentrically placed. (It is interesting to observe 

 that these cells are identical in form and size with those described and figured by 

 Polejaeff in the cortex of his Cacospongia vesiculifera} We are, however, rather 

 surprised that Polejaeff should speak of the cells in question as " thoroughly identical 

 with the vesicular cells of many Desmacidonidse — undescribed indeed hitherto, but 

 undoubtedly very well known to every spongiologist who has had to deal with the repre- 

 sentatives of the family just mentioned." So far as our experience goes we know of no 

 such vesicular tissue in the Desmacidonidfe.) In the outer portion of the cortex, 

 embedded in the layer of vesicular tissue, there occur also numerous, deeply staining, 

 nucleated, granular cells (PL LI. fig. Ih, g.c.) of irregular shape, of about the same size as 

 or a little smaller than the vesicular cells. These are, for the most part, arranged in not 

 very regular groups (PL LI. fig. 1, g.c). It appears very possible that, like the somewhat 

 similar cells in Stylocordyla, they have a glandular function. This layer of vesicular 

 tissue with its embedded groups of glandular (?) cells gradually gives place below to a 

 gelatinous looking tissue (in which the vesicular cells are still abundant), which, as it 

 approaches the choanosome, becomes more or less strongly but irregularly fibrous. The 

 lower two-thirds (roughly speaking) of the cortex are strengthened and supported by 

 abundant spicules, as already mentioned. 



In the genus PheUoderma, nobis, which forms a great exception to the rule that the 

 Halichondrina are never corticate, the ectosome is about 0'25 mm. thick, and the 

 fibrous tissue is concentrated in a zone of varying thickness in its lower part ; unfortun- 

 ately the histological condition of the single specimen at our disposal is not sufficiently 

 good to allow of a more detailed description. 



To sum up our observations on this head, we may briefly tabulate the chief different 

 modifications of the ectosome in the Monaxonida as follows : — 



Ectosome. 



Thin; 



forming only a dermal 



membrane (e.g., Halichondria, 



Raspailia, Phakellia, 



Esperiopsis, &c). 



Thick 



Gelatinous, 



commonly with an 



external, separable dermal 



membrane (e.g., EqKrella, 



SpirastreUa). 



Tough and fibrous, 

 sharply marked off from 

 the choanosome, no separable 

 dermal membrane (e.g., 

 Suberitidae. 



1 Zool. Chall. Exp., part xxxi., Report on the Keratosa, p. 59. 



(ZOOL. CHALL. EXP. — PART LIX. 1887.) 



Nnn e 



