ON THE DEVELOPMENT OF LINGULA ANATINA. 27 



appearance. The muscle originates on the ventral body wall at 

 the right and the left of the posterior end of the oesophagus, and 

 runs forward along the latter, disappearing in the arm-sinus at 

 about the same level as the mouth. It is indeed due to the 

 contraction of these muscles that the arm-apparatus is retracted. 

 It is frequently observed that an embryo when fatigued does not 

 contract its arm, and this may be due to the fact that these 

 muscles partly lose the power of contraction. These muscles 

 attain a high degree of development in later free-swimming larvœ, 

 and afterwards gradually degenerate. The mantle and shells show 

 a rapid growth as compared with the body proper and the arm- 

 apparatus. The marginal parts of the mantles are somewhat 

 thickened owing to the large cells composing them and to the 

 presence of the marginal lacuna (Randlacune) within. At this 

 stage both ends of the straight posterior edge of the shell terminate 

 in rounded angles, not in teeth as in embryos a little older. 



I have thus far made no mention of the general appearance 

 and color of the embryos. It is, therefore, not superfluous to 

 touch next upon those points. 



The embryo in general is transparent, but some thicker por- 

 tions have a slight opacity. In young embryos the cells are so 

 large that we can distinguish them individually. Hence they 

 look very granular. The mantle even in advanced embryos has 

 a mottled appearance in consequence of the unequal thickness of 

 the cell layers. In older embryos the arm-apparatus, especially 

 the tip of the cirri, is rich in refracting granules. The color 

 of the embryo is a light yellow with a greenish tone ; the thicker 

 part, of course, being more greenish and darker in color (PI. III., 

 Fig. 53.). In some of the oldest embryos which I was able to 



