114 TOILERS IN THE SEA. 



two series ; the first appropriated to the inflowing 

 (iricurrent) streams of the surrounding water, and 

 the second to the outflowing (excurrent) streams, 

 which they conduct from the interior of the sponge 

 to the oscula at its surface, through which they are 

 discharged. Enveloping the entire mass of the 

 sponge we find the dermal membrane, in which are 

 situated the pores for inhalation and imbibition of 

 nutriment, and the supply of the inflowing canals, 

 and the oscula, through which the excrementitious 

 matter, and the exhausted streams of water, are 

 poured from the terminations of the outflowing 

 canals. These parts are indispensably necessary, 

 and are always present in a living sponge. The 

 attachment of the sponge to the body to which they 

 adhere during life is effected by a basal membrane, 

 which presents a simple adhesive surface, following 

 the sinuosities of the body on which it is based, 

 entering into holes, or cracks, and filling them up, 

 thus securing a firm hold of the mass on which they 

 are fixed." 



The size and external form of sponges vary con- 

 siderably, even in the same species ; in the majority 

 there is such a delightful irregularity as to baffle 

 description. What the extreme size may be it is 

 very hard to conjecture > but Neptune's Cup is some- 

 times three feet high, and there are minute species 

 not so large as a pea ; between these two extremes 



