SHIP WORMS. 245 



of their coalesced syphons, which are covered with a 

 fringe of cilia that guard against the entrance of 

 unsuitable matter. Both species of these common but 

 elegant shells are kept in our public aquaria. In ad- 

 dition to the stone-borers (Saxicava and P kolas) at 

 the Crystal Palace may be seen the wood-boring 

 mollusca (Xylophaga and Teredo). The latter goes 

 by the common name of " ship worm," but it is in 



Fig. 177. 



Mactra stultorum. 



reality a mollusc in which the shells are reduced to a 

 minimum of size, the lime secreted by the animal 

 being used to line the winding worm-like burrows it 

 makes in the wood where it takes up its abode. The 

 Madras, Tellinas, Donax, and Venus are other prettily- 

 coloured and elegant aquarium mollusca. Cyprina 

 islandica, Modiola, Modiolus (the latter usually called 

 the " horse-mussel"), Pinna pectinata, Lima hians, 

 Scrobicularia piperata, &c., are many of them larger 



