THE SPONGES 



from the left branch of the former grows a flat alga incrusted by another sponge of 

 the same order, and of a dull yellow color; and at the top of the colony is the violet 

 Spongelia. To the left of p. 3517 is illustrated another sponge of this group 

 {Axinella), common in the Mediterranean. In life this sponge is of a yellowish 

 color, and the oscules present a radiate arrangement like the polyps of a branch 

 of fan coral. The accompanying illustration illustrates a sponge dredged by the 

 Challenger, and known as Esperiopsis challengeri, from six hundred and thirty 

 fathoms, east of the Celebes. The largest specimens are about eight inches in 

 height. From a solid, strong stem six or seven stalks are given off at gradually 

 increasing intervals from below upward, and each bearing a hemispherical cup or 

 ladle, convex outward. 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 three or four feet. It includes also the boring sponges (Ctiona*), 

 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 Professor 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. 3519. 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 

 of the action of a burrowing sponge, and serves to 

 distinguish the cavities and hollows due to the sponge 

 from those caused by worms, mollusks, 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, 



Esperiopsis challengeri. 

 (One-half natural size.) 



