BRITISH SPONGIAD^. 839 



diameter of the shaft of the spiculum in width ; but this 

 full amount of development varies in some spicula to that 

 of scarcely an indication of its presence on the sides of its 

 shaft. The fimbriae are not connected with the dentse of 

 the auchorate spiculum. They originate at the bases of 

 those organs, and the outer margin of each, curves gradually 

 outward until it reaches to near its termination ; it is then 

 rapidly inflected inward until it arrives at the shaft ; its 

 greatest expansion being at about four fifths of its length, 

 from its origin near the dental end of the spiculum. 



The retentive spicula of both sizes are very abundant 

 in the specimen from Mr. Barlee ; and they are remarkably 

 illustrative of the general character and laws of development 

 of these organs ; for in each set, their progressive growth 

 from their earliest bihamate condition to the adult form 

 may be readily traced by careful observation. The fim- 

 briation of the shaft of the spiculum is remarkably delicate, 

 and requires a careful manipulation of the light to render 

 it distinctly apparent with a linear power of about 600. 



Spongilla, LinncBus. 



1. Spongilla Jluviatilis, Johnston. 



2. — lacustris, Johnston. 



Spongilla Linn^us, Lamarck and Johnston. 

 Halichondeia, Fleming. 



1. Spongilla fluviatilis, Johnston. 



Sponge. Massive, sessile ; surface uneven, often lobular, 

 hispid. Oscula simple, large, scattered. Pores con- 

 spicuous. Dermal membrane pellucid, aspiculous. 

 Skeleton. Spicula acerate. Ovaria sub-globose ; spi- 



