STRUCTURE AND ECONOMY OF TETHEA. 413 



slightly granular but transparent substance, in which are 

 embedded numerous nucleated cells. These cells are gene- 

 rally acutely spindle-shaped, or split at one or both extremi- 

 ties. They are more abundant around the water-orifices, and 

 are there intermixed with many stellate and oi-stcllale silicious 

 spicula. 



The inner part of the rind is denser, and consists almost 

 entirely of nucleated fibre-cells, which run more or less trans- 

 versely, or across the axes of the spicula. 



Similar fibre-cells are met with abundantly in the fleshy 

 walls of the larger water-canals of their intermediate substance, 

 along with stellate and bi-stcllatc silicious spicula. 



The anatomical characters, arrangement, and relations of 

 the fibre-cells of Tethea, taken along with the acknowledged 

 contractility of the group, indicate the functions of the fibre- 

 cells in the economy of these sponges. Their peculiar aspect, 

 elongated form, transverse relation to the spicula, and grouping 

 around the water-canals, appear to me to indicate very clearly 

 their muscular character. I do not, however, conceive them 

 to be the only contractile structures in the mass of the sponge ; 

 for I am inclined to believe that the slightly granular gelati- 

 nous matrix, in which they and the other structures are em- 

 bedded, is also capable of a slow but decided change of form, 

 like the mass of an ama?ba, or the peculiar texture with 

 which observers are now familiar in so many of the lower 

 organisms, and which has been termed by Dujardin, Sarcode. 



The more energetic contractility of the fibre-cells would 

 appeal to be concentrated on the spicula and skeleton ; the 

 movements of the gelatinous matrix probably regulate the 

 diameter and capacity of the water-canals. 



Pores, Water -Passages, and Oscula. — The pores, or 

 afferent water-orifices, are arranged in groups of three, each 

 group in relation to a six-radiate spiculum, so that a pore is 

 situated in each angle between the three primary branches of 



