162 ANATOMY AND PHYSIOLOGY 



if the sponge be carefully split open and immersed in fresh 

 cold sea-water, and examined with a power of about five 

 or six hundred linear by transmitted light. The cilia will 

 be seen in rapid action just within the oscula which termi- 

 nate each of the large angular interstitial cells of the 

 sponge. This action, and the mode of the disposition of 

 the cilia within the cells, I have described at length in the 

 ' Transactions of the Microscopical Society of London/ vol. 

 iii, p. 137, pi. xix. In accordance with these variations 

 in structure I purpose dividing the British species into four 

 genera. 



CLASS P R I F E R A, Grant, 

 ORDER I. CALCAREA. 



Genera. GRANTIA. 



LEUCOSOLENIA. 



LEUCONIA. 



LEUCOGYPSIA. 



GRANTIA, Fleming. 



Sponge. Furnished with a central cloaca, parietes con- 

 structed of interstitial cells, more or less regular and 

 angular in form, disposed at right angles to the exter- 

 nal surface, and extending in length from the outer to 

 very near the inner surface of the sponge, where each 

 terminates in a single osculum. 



Type, Grantia conipressa, Johnston. 



The cloaca varies in its form and proportion. In some 

 species it has invariably one large terminal mouth, while in 

 others it is furnished with several mouths from which the 

 excurrent fsecal streams are discharged. 



The interstitial structures of the sponges of this genus 



