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- 

 pear to be the true oscula of the sponge." (Bowerbank, British 

 Sponges, vol. i. pp. 176, 177.) 



This is a gratuitous assumption, and seems improbable. 

 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 in a 

 fixed organism of the form of EuplecteUa^ 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 sm-ely 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 I'Astrolabe,' 

 and now in the Museum of the Jardin des Plantes. It is 

 represented (PI. IV. fig. 2) reduced one-third, from a photo- 

 graph, of the natural size, by M. Potteau. 



The specimen is labelled '■'■ Alci/oncellum corhicula^ Val. 

 Tire par 80 brasses de profondeur dans la rade de St. Denis de 

 Bom-bon par M. Leschenault, 1819." 



EXPLANATION OF PLATE IV. 



Fig. 1. Habrodictyon corbicuin, reduced one-third. 



1 a. One of the distorted hexradiate spicules, X 100. 



1 6. A regular hexradiate spicule, x 2-50. 



1 c. One of the ordinary filiform spicules of the skeleton, showing the 



tubercles which represent the secondary rays, x 150. 

 1 d. The enlarged end of such a spicule. 



1 e. A portion of one of the " floricomo-hexradiate stellate" spicules, 

 X800. 



1 /. One of the separated branches, front and lateral views, X 1000. 

 Fiff. 2. Habrodictyon speciosunij reduced one-third. 



2 a. One of the spicules of the sarcode peculiar to this species, X 1000. 



