ON THE DEVELOPMENT OF LINGULA ANATINA. 31 



4. Mantle. 



In the embryo the mantle is formed at a very early period 

 as a single ring-shaped fold or ridge, around the arm-ridge (PL 

 IV., Figs. 55, 56.), and is afterward cut up into two lobes as 

 has been described before. Since it is formed as a fold of the 

 ectoblast the mantle consists of two layers, each layer being one 

 cell thick. The inner layer is very thin while the outer attains 

 a considerable thickness (PL IV., Figs. 00, 61, 64-68.). The 

 mantle lobes grow not only at the anterior and lateral, but also 

 at the posterior edge, where growth takes place in two parallel 

 ridges, leaving a shallow furrow between them (PL IV., Figs. 

 60, 61. f\). The epithelium forming the above furrow becomes 

 very thin (PL IV., Fig. 64.). We perceive in the course of 

 development that the mantle increases in size faster than the body 

 proper and the arm-apparatus {Cf PL IV. Fig. 61. and Fig. 70.), 

 and becomes thinner and thinner as it grows, retaining its original 

 thickness only along its margin — a peculiarity which is due to the 

 presence of comparatively large cells and of the spacious marginal 

 lacuna (PL IV., Fig. 70.). 



In passing, I should like to state here that the ectoblast 

 constituting the lateral body wall is thicker than the inner layer 

 of the mantle, and each cell is highly rounded and vaulted 

 toward the exterior (PL IV., Figs. 65, 68.). 



5. Shell. 



A little prior to the rupture of the vitelline membrane a 

 thin cuticular layer is secreted on the dorsal and ventral surfaces 

 of the mantle. The shell is formed at first as a circular lamella 



