TYPICAL GROUP. 
543 
increasing intervals from below upwards, and each bearing a hemispherical cup or 
ladle, convex outwards. The in-current pores are on the concave, and the out-current 
on the convex surface. To this order belongs the Neptune’s cup sponge ( Poterium ), 
attaining a height of 3 or 4 feet. It includes also the boring-sponges ( Cliona ), in 
which the skeleton consists of scattered pin-shaped spicules. To compensate for 
the inefficient support, the sponge excavates into limestone, chalk, or shells. Great 
importance is attributed to the destructive power of 
these sponges by Prof. Schmidt, who points out that 
considerable portions of the coasts of the Mediter¬ 
ranean consist of limestone, the disintegration of 
which has been greatly hastened by the operation of 
boring-sponges. In many parts the outlines of the 
coast have consequently been much altered, and along 
the Dalmatian shore, for a distance of a thousand 
miles, one may find the beach thickly strewn with 
stones completely riddled with the holes made by these 
sponges, as shown in the illustration on p. 544. The 
causes of this property of the burro wing-sponges are 
not known; but there are two theories, mechanical 
and chemical. According to the former, the sponge 
bores by means of the grinding of its siliceous spicules 
against the softer limestone. The action would be 
assisted partly by the action of the contractile sub¬ 
stance of the sponge-body, and partly by the currents 
of water traversing the canals. On examining the 
galleries of a shell or piece of limestone with a lens, 
the surface is seen to be pitted with minute hemi¬ 
spherical cavities, giving rise to a finely shagreened 
appearance. The shagreen surface is characteristic 
of the action of a burrowing-sponge, and serves to 
distinguish the cavities and hollows due to the 
sponge from those caused by worms, molluscs, or the action of water. Certain 
minute five-sided plates were formerly supposed to assist in the excavating process, 
but are now known to result from the breaking down of the organic layers of the 
shell. On the other hand, the advocates of the chemical theory attribute the 
excavating properties to the secretion of carbonic acid by the sponge, which is 
thus enabled to dissolve the carbonate of lime of the shell or limestones; but an 
objection lies in the fact that carbonic acid is incapable of dissolving the organic 
plates of shells. Recently it has been urged that the power of contractility 
possessed by the sponge is a powerful aid in the work of excavation. Burrowing- 
sponges are a trouble in oyster-culture, and it is suggested that at the time when 
the free-swimming sponge-embryos are formed, a bank of old shells should be 
placed between the oyster-beds and the tide. The bank would filter ofi‘ the 
embryos, which would grow in the old shells, and be subsequently destroyed by 
immersion in fresh water. A figure of a fragment of limestone thus perforated by 
sponges is given on the following page. 
