CONCHIFERA. 317 



branchial leaflets of the parent, or the young are nursed 

 in a special pouch affixed to the gills until they are suffi- 

 ciently mature to take care of themselves, as in the genus 

 Spharium or Cyclas. The Conchifera being invariably 

 gill-bearing, all inhabit the water, in which element the 

 young fry undergo a kind of metamorphosis. At first they 

 swim freely about by means of a ciliated disk, at the fore 

 part of which is attached a slender, vibritile filament or 

 jlabellum. At this period of their lives the young Bivalves 

 are provided with a pair of eyes, situated near the mouth, 

 which afterwards, however, entirely disappear, and the adult 

 animals are totally blind, unless the so-called eyes which 

 are placed around the mantle-margin in some tribes, as the 

 Arcs and Scallops, can be considered as organs of vision. 

 In the next stage of their growth the swimming disk and 

 Jlabellum gradually disappear, the labial palps which sur- 

 round the mouth become developed, and a new organ of 

 locomotion, or foot, makes its appearance. Some of these 

 young Conchifera attain their full growth in the course of 

 a single year, as the Mytili and Cardia, while others would 

 seem to require an indefinite period for their complete de- 

 velopment; such, for example, must be the case with the 

 giant Tridacna, which lives in masses of madrepore, and 

 becomes gradually embedded in, and overgrown by, the 

 surrounding coral in the lapse of years. The embryonic 

 shell forms the nuclei or beaks of the valves, and often 

 assumes, as in the Gasteropoda, an entirely different appear- 

 ance from the same shell in its after- stages of growth. 



The mouth in the Bivalves is a simple, transversely oval 

 aperture, unprovided with jaws, without a tongue or armed 

 lingual ribbon, but furnished on each side with a pair of 

 soft, membranous palps. These labial palps guard the 



VOL. II. t T 



