THE DEVELOPMENT OF LAMELLIBRANCHS. 327 



point a little behind the mouth, and a second opening, the 

 anus (Fig. 171 and 172, an), is formed. The tract which 

 connects the anus with the stomach lengthens and forms 

 the intestine, and, soon after, the sides of the stomach 

 become folded off to form the two halves of the liver, as 

 shown in Fig. 172. 



Various muscular fibres now make their appearance 

 within the body, and the animal assumes the form shown 

 in Fig. 172. 



III. The Swimming Larva or Veliger. 



It is difficult to rear the embryos, but the later stages 

 may be studied from specimens collected with the dip-net. 

 The swimming larvae or "Veligers" of marine lamellibranchs 

 are so abundant at the surface of the ocean during the 

 summer months that there is no difficulty in obtaining a 

 supply. In order to find them, allow the material which 

 has been collected with the dip-net or the tow-net (as de- 

 scribed in Section VII.), to stand over night in a jar of 

 sea-water. Then draw up with a dipping-tube a little of 

 the sediment which has settled at the bottom, and placing 

 it in a watch-crystal with a little sea-water, examine it 

 with a magnifying power of about fifty diameters. A little 

 j-c.-i rdiing will probably lend to the discovery of several 

 of the larvae lying upon the bottom among the sediment, 

 tightly shut up in their transparent, orbicular, or kidney- 

 shaped shells. The student will recognize them without 

 difficulty, since the sholl is shaped much like that of the 

 ndult. Having found a specimen, carefully note its posi- 

 tion with reference to adjacent masses of sediment, and 

 then try to rediscover it without a microscope. Having 

 done so, push the sediment away from it with a hair, and 

 sucking the specimen up into a dipping-tube, transfer it to 

 a small quantity of fresh sea-water. Place it under the 



