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 burrowing-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 IB 



of the action of a burrowing -Sponge, and Serves to Esperio^sis challenge ( nat. size). 



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 off 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. 



