10 EXTERNAL FEATURES. 



rithmic spirals of Moseley, or conchospiral of Naumanii. Cor- 

 responding to the shell, which is pi'oduced by the mantle, and 

 borne by the posterior portion of the body, the posterior dorsal 

 part of the foot bears an operculum, secreted by an expansion of 

 its skin called the opercular mantle. Typically, the operculum 

 is a spiral also, but in the same plane ; yet in many cases its 

 growth is annular. Usually horny, it is sometimes entirely or 

 partially calcareous, and on the retirement of the animal within 

 the shell it is brought into the aperture of the latter, which it 

 more or less completely closes. Like the shell itself, it may be 

 considered a protective organ, and when in apposition with the 

 former suggests the two enclosing valves of the lamellibranchiate 

 or bivalve mollusks. 



Notwithstanding the large portion of the animal which is 

 always within the shell, even when the head and foot are 

 extruded, the latter is only attaclied to the former at one point, 

 on the columella, and by means of a columellar muscle, which, 

 passing through the foot, is attached at its other end to the 

 operculum (when the latter is developed, which is not always 

 the case). Although there is only this single actual bond of 

 connection between shell and animal, the contact of the body 

 serves to maintain the vitality of the shell, which soon bleaches, 

 and finally decays when separated from its architect and inhab- 

 itant. 



The Pelecypoda (xxii) differ from the preceding two great 

 classes of the Mollusca in having no specialized head, though 

 provided with a mouth, organs of sight, etc. The foot is a fleshy 

 process adapted hy its form to digging rather than to locomotion. 

 The gills^ arranged on either side the body, the enclosing valves 

 of the testaceous covering, which are united by a hinge dorsally 

 or at their initial point, the arrangement of those organs which 

 are not paired in the median plane of the body — all show that 

 the bivalve mollusk is symmetrical ; although in certain attached 

 genera the intestines are crowded into the lower side or valve, 

 the cavity of the valve uppermost m position being usually very 

 shallow. 



The security against many enemies which their completely 

 enveloping bivalve shell affords the pelecj^Doda, and the great 

 proportionate weight of this testaceous covering have tended to 

 produce in these animals a much less active existence than that 

 of the encephala ; their movements are slow usually, and the 

 adults of the attached families, such as Spondylus, Ostrsea, etc., 

 remain for life in one position without possessing the power of 

 changing their residence. The food of the pelec^^poda cannot 

 be said to be sought for, but is simply selected from such 

 vegetable and minute animal organisms as the water may float 

 within reach of their mouths. These generally sluggish habits, 



