278 FUNCTIONS AND INSTINCTS. 



also, the shell is multivalve, but the animal pro- 

 trudes itself at the sides, and has no operculum, 

 as in the common barnacle. 1 Others, again, are 

 protected by a shell consisting of two valves, 

 open at one or two ends, and these seek further 

 protection either by burying themselves in the 

 sand or perforating the rocks, or by suspending 

 themselves by a byssus ; others, again, which 

 only open their shells at certain times, as the 

 oyster, fix themselves to any convenient sub- 

 stance. To these succeed others, whose shell is 

 transversely divided into many pieces, 2 but yet, 

 taken together, it forms a single valve protecting 

 the back of a gastropod, or slug-like animal, 

 which for further protection, when it is not mov- 

 ing, and to supply the place of a lower valve, 

 fastens itself to a rock or other substance. 



With the Patellidans begin the undivided uni- 

 valve shells, which like the preceding animals 

 protect their lower side by fixing themselves 

 to the rocks ; the sea-ears, 3 which are still more 

 open, have recourse to a similar mode of pro- 

 tecting themselves, they preserve a communi- 

 cation with the atmosphere or water without 

 elevating their shells, by means of a line of 

 apertures, under the thickest margin near the 

 apex; these apertures begin when the animal 

 is young near the spire, and as it grows it stops 



1 Pentelasmis. 2 Chiton. 5 Haliotis. 



