THE SPONGES 37 



downpour of rain. The fresh rain-water kills the Sponges, but the 

 oysters close their shells and so escape injury. 



Many of the Encrusting Sponges cover the rocks in deep tidal 

 pools and are very beautiful in their rich colouring of red, yellow , 

 orange, etc. The cup-shaped fine Turkey Sponge (Spongia offi- 

 cinalis), the cake-shaped Common Horse or Bath Sponge (Hippo- 

 spongia equina), and the flat, disc-shaped Hard Sponge (Spongia 

 zimoced) are the three typical commercial species ; the Levant 

 Lappet, which forms great thin flaps like an elephant's ear, is 

 a variety of the Turkey Sponge. 



All the commercial Sponges live in sub-tropical and tropical 

 seas, at depths of 2 to 100 fathoms, the world's supply being drawn 

 almost entirely from the eastern half of the Mediterranean, and 

 from the West Indies. They are collected by divers, who descend 

 to the floor of the sea and detach them from the rocks ; or are 

 hooked up by means of a kind of long harpoon, or by dredges 

 in the deeper waters. These commercial Sponges are also 

 cultivated from cuttings, the time taken by a small cutting about 

 a cubic inch in size to grow into a marketable Sponge being 

 about seven years. 



Sponges appear to be very distasteful to other animals, and 

 to be eaten by very few. Consequently it is not altogether sur- 

 prising to find an intimate association or symbiosis existing between 

 them and other animals, more particularly crabs. Many of the 

 spider crabs cover themselves with pieces of Sponge, which they 

 attach to their body and legs, where the Sponge grows quite 

 healthily. Dromia, the sleeping crab, invariably holds a living 

 Sponge upon its back, certain of its legs having become modified 

 and adapted for the purpose, the Sponge growing and moulding 

 itself to the shape of the body, so that the crab, when at rest, is 

 completely concealed. Another Sponge (Sttberites) very commonly 

 grows upon the whelk-shell inhabited by a hermit crab, and soon 

 absorbs the shell, so that the hermit crab inhabits a cavity in the 

 Sponge ; while within many Sponges minute algae live in constant 

 partnership. 



The Sponges seem to have made their appearance very early 

 in the history of animal life, as one would expect from their com- 

 paratively lowly organisation ; their fossil remains have been 

 found in the Cambrian strata and in the strata of each succeeding 



