ANATOMY OF THE PEARL OYSTER. 



43 



nacre), and that is the point to which the mantle is retracted in preserved specimens, 

 leaving the non-nacreous part of the valve exposed (see also text-fig. 1, where the 

 pigmented mantle edge is drawn up on the left side of the figure to show the nacre). 



The mantle has the same general structure as in other better-known Lamellibranchs, 

 such as the European oyster, Ostrea edulis) It is composed mainly of connective- 

 tissue traversed by muscular bundles and numerous blood spaces, and covered on both 



Fig. 1. Dissection of pearl oyster from the right side, showing the stomach and digestive gland, 

 the gills, foot and byssus, pigmented pallia] margin, and a cyst-pearl m situ at the top left- 

 hand corner ; natural size, from a photograph by Mr. HORNELL. 



outer and inner faces by a layer of epithelial cells, the ectoderm (Plate VIII., fig. 2). 

 The epithelium on the outer surface, next to the nacreous layer of the shell, is 

 secretory, while on the inner free surface, facing the pallia! cavity, the ectoderm is 

 ciliated (Plate VIII., fig. 2, i.ep.). The centre and dorsal part of each pallial lobe is 

 adherent to the rest of the body and thus envelops the viscera (Plate V., fig. 10, Pall.), 

 while the ventral and marginal parts hang down freely like a flap or curtain, so as to 

 form the side walls of the pallial cavity (see various sections on Plate V.). 



Further details of the minute structure of the mantle are seen in fig. 2 on 

 Plate VIII. The marginal and velar processes, the deep periostracal groove, the 

 blood spaces, the glands, the muscles, and the nerves are all evident. The character 

 of the epithelium in different parts is shown in the more highly magnified side 

 figures: A. shows the tall epithelium which secretes the prismatic layer; B. is the 

 much lower general surface which deposits nacre ; C. shows the ciliated internal 

 surface ; D. and E. the side and end of the marginal groove, with glands and high 

 epithelium for the secretion of the periostracum. 



G 2 



