‘132 ~~ Prof. W. Thomson on the ‘ Vitreous”. Sponges. 
Dr. Bowerbank supposes that in this group the openings of 
the lid and those of the tube will stand to one another in the 
relation of oscula and pores: ‘‘'The whole of the parietes are 
appropriated to inhalation.” The distal end of the cloaca “is 
partially closed by a cribriform veil, the orifices of which ap- 
ear to be the true oscula of the sponge.” (Bowerbank, British 
ponges, vol. i. pp. 176, 177.) ay 
his is a gratuitous assumption, and seems ei pee? 
Even in Euplectella, in which the formation of the lid is most 
perfect, the meshes of the tube-wall are individually as large 
as the openings in the lid, and collectively represent an area 
of a hundred times their extent. It seems to me that ina 
fixed organism of the form of Huplectella, with so open a 
structure, the resistance at the contracted “‘oscular area” 
would be sufficient to overcome any ciliary current concen- 
trated upon it, and to send the water back through the open 
network. It is surely much more likely that each of the large» 
openings in the wall is occupied by an exhalant orifice, and 
that inhalation takes place as usual by minute pores in the 
interstices between the spicules of the skeleton. Indeed this 
is scarcely an open question; for in the unique specimen of 
H. speciosum there is no lid, and the apertures are of the same 
character throughout. | 
The only known specimen of H. speciosum is that figured 
by MM. Quoy and Gaimard in the ‘ Voyage de |’Astrolabe,’ 
and now in the Museum of the Jardin des Plantes. It is 
represented (Pl. IV. fig. 2) reduced one-third, from a photo- 
graph, of the natural size, by M. Potteau. 
he specimen is labelled ‘ Alcyoncellum corbicula, Val. 
Tiré par 80 brasses de profondeur dans la rade de St. Denis de 
Bourbon par M. Leschenault, 1819.” 
EXPLANATION OF PLATE IV. 
Fig. 1. Habrodictyon corlicula, reduced one-third. 
la, One of the distorted hexradiate spicules, x 100. 
16. A regular hexradiate spicule, x 250, wees 
lc, One of the ord‘nary filiform spicules of the skeleton, showing the 
tubercles which represent the secondary rays, x 150. 
1d. The enlarged end of such a spicule. 
leA en of one of the “ floricomo-hexradiate stellate ” spicules, 
x 800. 
1 f. One of the separated branches, front and lateral views, x 1000. 
Fig. 2. Habrodictyon speciosum, reduced one-third. 
2 a, One of the spicules of the sarcode peculiar to this species, x 1000. 
