Sponge Fisheries of the Mediterranean. 195 



seas, though the commercial ones are not known to have 

 existed earlier, because the keratose matter furnishes 

 hardly favourable conditions for petrifaction. In the oolite 

 and chalk formations, sponges containing flinty spicules 

 were very abundant ; and in most of the earlier formations, 

 large sponges containing calcareous spicules abounded. 

 These very closely resemble corals, and have been mistaken 

 for them by some of our best geologists. The spiculae or 

 needle-shaped particles, which are often microscopic in size, 

 are not thrown in without order, but are arranged to support 

 the skeleton. The horny sponges do not secrete or deposit 

 spicules, but these are sometimes found within the skeleton 

 in broken and disordered form, which show they were 

 taken in from without. 



The quantity of silica which constitutes the structure of 

 sponges is remarkable. It generally occurs in the form of 

 spiculae in considerable quantities, imbedded in the sub- 

 stance or body of the sponge. One of the rarest and most 

 beautiful of the silicious sponges is the Euplectella speciosa, 

 found at the Philippine Islands. It is of cornucopia shape, 

 and has a horny, skeleton-like network, composed of large 

 silicious fibres running from the base to the head, sur- 

 rounded by small fibres forming square, open meshes, 

 resembling a net or basket work. It ranges in height from 

 6 to over 1 5 inches. At the lower extremity, or root, it 

 averages about an inch in thickness, but its size gradually 

 increases as it approaches the top, where often it is two 

 inches wide. It is surmounted by a ridge about a quarter 

 of an inch wide, and is closed at the larger extremity by a 

 delicate open lacework of fibres, possessing no particular 

 pattern. It is on this light and pretty structure that the 

 fibrous, gelatinous substance rests, resembling in texture 

 the common sponge, but in this instance disposed in an 



