354 TOILERS IN THE SEA. 



with a species of sponge, found only in such a posi- 

 tion (fig-. 69). Whether or not the sponge itself has 

 bored the holes, and excavated the chambers, some 

 have ventured to doubt, but that the sponge called the 

 " Boring sponge " or " Burrowing sponge " is always 

 found lining these cavities, is sufficient to justify us 

 in treating it as an excavator. Dr. Leidy gives a 

 full and particular account of the sponge in its living 

 state. " This boring sponge," he says, " forms an 

 extensive system of galleries between the outer and 

 inner layers of the shells, protrudes through the per- 

 forations of the latter tubular processes,, from one 

 to two lines long, and one-half to three-fourths of a 

 line wide. The tubes are of two kinds, the most 

 numerous being cylindrical, and expanded at the 

 orifice, in a corolla form, with their margin thin, 

 translucent, entire, veined with more opaque lines, 

 and with the throat bristling with siliceous spicules. 

 The second kind of tubes are comparatively few, 

 about as one is to thirty of the other, and are shorter, 

 wider, not expanded at the orifice, and the throat 

 unobstructed with spicules. Some of the second 

 variety of tubes are constituted of a confluent pair, 

 the throat of which bifurcates at bottom. Both kinds 

 of the tubes are very slightly contractile, and, under 

 irritation, may gradually assume the appearance of 

 superficial, wart-like eminences, within the perfora- 

 tions of the shell occupied by the sponge. Water 



