R. W. H. ROW—REPORT ON THE SPONGES : NON-CALCAREA. 355 
sponge, and one can only suppose that they are foreign bodies taken up by 
this sponge as others take up sand-grains for the same purpose. A noticeable 
difference between the two cases occurs, however, in the fact that these 
calcareous spicules occur well below the dermal membrane, and not as a 
superficial layer at the surface of the sponge. 
A careful examination of the whole specimen has entirely failed to find 
any animal at all which might have produced these spicules, 
but Mr. Crossland’s notes (quoted below) show that com- 
pound Ascidians occur here as well. 
Spicules. (Text-fig. 24.) 
The spicules of the sponge (omitting the dermal cal- 
careous asters) are all tylostyles, with usually a straight 
shaft and well-developed oval head. The shaft tapers very 
: | gradually from its thickest part, which is about the middle 
of the spicule, towards the head, and rather more sharply 
mt (though still gently) to the pointed end. 
| | The spicules vary from 0°1 mm. to 0°15 mm. in length, 
| and average about 0:0015 mm. to 0:0025 mm. in 
| diameter. 
Caleareous asters. 
ae ee, These bodies vary very much in the number of rays they 
19. 24. . 2 : : 
BAL Wee. possess, some only possessing 6 or 8 rays, while in others 
Hymeniacidon 
oleae there may be 50 or even more. They are much more like 
Spicules, x 350, some of the calcareous asters of some Ascidians than like the 
siliceous asters of the Astrotetraxonida. They vary in 
diameter from 0-016 mm. to 0:022 mm. 
Locality. “From the under side of a buoy in Suakin Harbour. They, 
with Compound Ascidians and Barnacles, here enter into competition with 
M. vulgaris (Pearl Oyster) and prevent its attaining a profitable age.” 
Distribution. Red Sea. 
HYMENIACIDON ZOSTER, n. sp. (Text-fig. 25.) 
This new species is represented by several specimens, all growing on the 
stems of Zostera plants, and usually at the nodes. 
The largest specimen is a small, rather irregular, but somewhat rounded 
mass, which is growing in a fork of the stem, and evidently gradually 
spreading up and down the stems of its support. The greatest diameter of 
the mass is about 25 mm. 
The surface is smooth, but here and there spicular fibres project, which 
are easily visible to the naked eye, and which may project a considerable 
distance. 
The oscula are numerous but small, and do not measure more than 
