MOLLUSC A. 33 



time subsequently, much more symmetrical than the 

 adult afterwards becomes. 



Even such forms as the scallop, which are not only 

 free but capable of migrating at different seasons of the 

 year from deep to shallow water, nevertheless habitually 

 lie on one side, and show the effect of this habit in the 

 inequality of the sides. This animal — Pecten, or 

 Comb-shell, or Scallop — is also remarkable in the 

 possession of a foot, which, however, is not used as an 

 organ of locomotion, and in having a row of eyes situ- 

 ated among the fringes on the edges of the mantle. 



These can be obtained of fish-dealers, but they must 

 be cautioned to get them alive and whole, since, when 

 cleaned for sale, everything is thrown away except the 

 large central muscle, which alone is considered suitable 

 for food. 



Fig. 1 7 gives a general idea of the distribution of the 

 parts and the situation of the eyes {ey^ , which last have 

 lenses and special nerves, and are probably capable of 

 seeing distinctly. 



The gills are particularly interesting, because they 

 show the intermediate stage between the single gills of 

 Nucula or Leda, and the completed gills of the oyster, 

 clam, etc. The gills in Nucula and Leda are, in their 

 simplest form, rows of tubes on either side of two fleshy 

 bases or arms projecting from the mantle, as in Pecten, 

 but somewhat longer. Near the foot a section in 

 Leda (Fig. 19) shows that organ (/^) with its split 

 muscular ridge, effectually used for crawling in the soft 

 mud in which they live,* and the tubes (^) on either 



* It is thrown out with edges together, penetrating the mud 

 easily, and then quickly spread out as in the figure, taking hold 

 and enabling the animal to move itself rapidly forward. 



