Mr. H. J. Carter on the Sithsjjherous Sponges. 13 



sarcocle, of about two-fifths the entire thickness of the dermal 

 crust, which is permeated by numerous minute canals, which 

 convey the water inhaled by the pores to the expanded distal 

 extremity of the cavity. The proximal end is closed by a 

 stout membranous valvular diaphragm, which the animal has 

 the power of opening or closing at its pleasure." 



Now, the result of my dissection of this structure, both in 

 Geodia and Pachymatisinaj being somewhat different and more 

 elaborate, it will be better to describe it in my own words ; 

 and using the term " pores " in the sense of Dr. Johnston, viz. 

 for the dimpled depressions of the sm'face, it is perfectly evi- 

 dent that they are the orifices of hourglass-shaped openings in 

 the crust, whose constricted portion is situated about midway 

 betu^een the external and internal surfaces of the latter, as 

 proved by their expanded portions on either side requiring to 

 be scraped off for a better observation of the constricted one. 



These hom'glass-shaped openings are lined throughout with 

 a thin film of sarcode, which, in the constricted portion, still 

 further reduces the diameter of this part by extending itself 

 across it in the form of a diaphragm provided with a central 

 opening which is more or less spiral tnicards, the outer part 

 of the diaphragm being always Jlaf. Moreover the spire, 

 which commences in the aperture of the diaphragm, is some- 

 times prolonged inwards from it in the form of a spiral tube 

 of four or five turns, which is again constricted in the centre 

 and free at the -itiner extremity — thus dipping as it were into 

 the inner portion of the hourglass cavity. (PI. II. figs. 11, 12.) 



Hence the aperture through the diaphragm is more or less 

 spirally continued on on its inner side. 



Inwardly the film of sarcode lining the inner portion of 

 the hourglass opening of the crust is in continuation with that 

 lining the areolar or vacuolar cavities situated at the circum- 

 ference of the parenchyma of the sponge, into one of Avliich 

 this part of the hourglass opening expands itself; and here, 

 at the commencement of the expansion, may be observed mi- 

 nute apertiu'cs, which are more or less scattered all over the 

 surface of the areolar cavity. Some of these appear to be in- 

 tended to keep up communication between the adjoining areolar 

 cavities, while others, viz. those on the vault or portion next 

 the crust, are the terminations of certain canals coming from 

 the surface of the sponge, to be hereafter mentioned. 



Externally the hourglass opening is covered by the dermal 

 sarcode when this is present, which is not always ; for it is 

 frequently absent in parts, having probably been i-ubbed off 

 by the rolling about of the free specimens in the sand at the 

 bottom of the sea ; but whether present or absent, the hour- 



