THE COMMON SPONGE. 389 



Halichondria celata establishes itself in the small holes which 

 some of the smaller annelides drill in the shell of the oyster, 

 eat further and further into the unfortunate mollusc's vitals, 

 causing the softer parts of the shell ,to rot away, and spread 

 through its whole substance, like the dry-rot fungus through a 

 Bolid beam of timber, until, sinking under the weight of his 

 misery, the poor victim perishes, and his loosened shell is cast to 

 the mercy of the waves. On the other hand, some marine Acorn- 

 ehells nestle habitually in a sponge, the normal construction ol 

 the base of the shell being altered to suit the peculiarities of its 

 habitation, so that in this instance, as in many others, there- is a 

 foreseen relation between two very dissimilar animals. Amongst 

 the reticulated fibres of its spongy dwelling, the Acorn-shell 

 finds a secure refuge in its infant state, and is soon enclosed by 

 the growing fabric of the sponge-animal, except a small opening, 

 which is kept clear by the vortex occasioned by the constant 

 motion of its feelers or tentacula. 



But very few of the manifold species of sponges are of any use 

 to man. The common sponge of commerce {Spongia communis), 

 so serviceable in our households, is most abundant in the Lycian 

 seas, where it is found attached to rocks at various depths 

 between three fathoms and thirty. When alive, it is of a 

 dull bluish black above, and dirty white beneath. There are 

 several qualities, possibly indicating as many distinct species. 



" The most valued kinds," says Edward Forbes, " are sought 

 for about the Gulf of Maori, along the Carian coast, and round 

 the opposite islands. The species which live immediately along 

 the shore near the water's edge, though often large, are worthless. 

 These are of many colours; some of the brightest scarlet or 

 clear yellow form a crust over the faces of submarine rocks ; 

 others are large and tubular, resembling holothurise in form 

 and of a gamboge colour, which soon turns to dirty brown when 

 taken out of the water; others are again lobed or palmate, 

 studded with prickly points, and perforated at intervals with 

 oscula. These grow to a considerable size, but, like the former, 

 are useless, since their substance is full of needles of flint." 



Large quantities of excellent sponge (Spongia usitatissima} 

 are likewise imported from the West Indies. 



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