44 CEYLON PEARL OYSTER REPORT. 



Each pallial lobe may be divided into three partsi a central, a distal or muscular, 

 and the marginal mantle-edge. The central pallial area extends from the mid-dorsal 

 line to the pallial line (Plate IV., fig. 1). where the shell is marked with muscle scars. 

 This part of the pallial lobe is perforated by the insertions of the adductor (Add.), 

 the retractor (Ret.), the levator (Lev.p., Lev.a.), and the pallial muscles. In a healthy 

 condition of the living pearl oyster, the tissues of this part of the mantle are soft and 

 mucoid in consistence, and opalescent white in colour. After hardening in alcohol 

 this tissue becomes brittle, and has a semi-prismatic fracture which is very charac- 

 teristic. All varieties of pearls cyst, ampullar, and muscle pearls may form within 

 its substance. Text-fig. 1, p. 43, shows a cyst-pearl in situ close to the dorsal 

 extremity of the adductor muscle. 



The distal or muscular area is translucent, and is capable of considerable contraction 

 by its muscles, and distention by the influx of blood into the large sinuses it contains. 

 It is formed of a thick layer of loose connective-tissue traversed by nerves and blood- 

 spaces, and by the radiating fan- like bundles of the pallial muscles (Plate VI., 

 figs. 1 and 14, Ret. Pal!.). This region is highly sensitive and irritable, and so con- 

 tractile that we found it difficult to introduce even minute foreign bodies between the 

 mantle-lobe and the shell in our experiments on artificial pearl-production. 



The marginal region or mantle-edge is chiefly a muscular thickening which ends in 

 two thin membranous folds with pigmented papillate edges (Plate II., figs. 3, 5, a, 

 and text-fig. 1). The outer of these, bearing digitate papillae, is in the same plane as 

 the inner surface of the shell and forms the true pallial edge (Plate V., fig. 1, Mg.Pall.). 

 The inner, which bears flattened palmate papillae, and may be called the pallial veil or 

 velum, projects inwards at right angles from the mantle-edge (Plate V., fig. 1, Vel.), so 

 that the veil of the one pallial lobe stretches towards that of the other(Plate II., fig. 5, a). 

 In life the free edges of the two veils are usually in contact along the median line of 

 the body, except at two spots where they gape. One of these, the inhalent aperture, 

 is somewhere about the middle of the ventral surface ; while the other, the exhalent, 

 is at the posterior end, opposite the opening of the anal funnel and supra-branchial 

 chamber. The former is not a permanently localised or specialised opening, its position 

 and its size and shape vary considerably from time to time, so that any part of the 

 ventral edge may form temporarily the inhalent gap through which the main in-flowing 

 current passes. The exhalent aperture, on the contrary, is definitely localised and 

 specialised. In outline it is broadly ovoid or almost circular (Plate III., fig. 10). Its 

 broad rounded lower (ventral) margin is immediately dorsal to the pallial fold, an 

 inwardly directed gutter-like fold of the velar margin that meets the tip of the 

 ctenidium (see Plate VI., figs. 1, 2, &c, Pall.f.). 



In 3 to 4-year old oysters the velum has a breadth of fully -j- inch on the ventral 

 aspect where it is most fully developed. Here, too, the marginal processes are largest, 

 and are of two kinds, long and short, several of the latter usually alternating with 

 the singly placed longer (Plate III., tig. 6). Diversity of form in the larger ones is 



