MARINE UNIVALVES. 235 



stead of external, dates back no further, perhaps, than 

 the oolitic epoch. 



Shell fish, both univalve and bivalve, are kept 

 in aquaria, according to their habits ; although the 

 limpet, chiton, fissurella or " key-hole limpet," dog- 

 whelk, and periwinkle are peculiar to shallow waters, 



Fig. 165. 



Animal of Fissurella. Key-hole Limpet 



(Fissurella Graca). 



and will creep out into the air. These are most 

 of them easy of domestication in such shallow tanks 

 as an amateur would commence with. All are in- 

 teresting objects, the limpet family, including the 

 several species of chiton, being particularly so. The 

 latter looks like a woodlouse, and has the power 

 of partially coiling itself up when detached from 

 the rock, to which, however, it clings as long as 

 it can with all the proverbial force of a limpet. 

 Formerly it was called a " multivalve " shell, on 

 account of this latter being made up of a number 

 of transverse pieces. It is, however, a near rela- 

 tive of the limpet. Their young are shell-less, and 

 have a semi-ring of cilia around the margin of the 



