THE CEYLON PEARL-OYSTER. 307 



due to simple secretion, arises by a gradual transformation into 

 shell-substance of the outer regions of the specialised epidermal 

 cells which underlie it. i. e. in the same manner as the Crustacean 

 carapace and the outermost layer of the periosti-acum. It shows 

 a well-marked striation perpendicular to the surface, the stria} 

 no doubt corresponding to the outlines of the spaces which were 

 occupied by the needle-like fibi'ocrystalline bodies of calcium 

 carbonate, and also at times indistinct lines parallel to the surface 

 (PI. XXXIV. fig. 5). In sections in the plane paridlel to the 

 surface this substance shows an alveolar structure. It sometimes 

 shows a tendency to break up into segments, corresponding to 

 the underlying epithelium-cells (PL XXXIY. fig. .5, ht/.'). 



(v.) 11 inye- Ligament. 



I do not propose to discuss the hinge-ligament here, as it has 



not the same direct bearing on the question of pearl-formation 



as the above layers, though leathery pearls, composed of this 



substance, are sometimes found (e. g. in M. maxima in Australia). 



(10) The Shell-secreting Epithelia. 



The oidinary shell-secreting epidermis of J/, vulgaris, so far 

 as I have been able to study it in the unsatisfactory material 

 available, consists of columnar or tesselated cells (PI. XXXV 

 &g.8,ep.; fig. 9, o.ep. ; PI. XXXVI. fig. 11, o.ep.; PI. XLI.' 

 lig. 33, ep.), brick-shaped or palisade-like, according to the 

 degi-ee of contraction, in sections perpendicular to the surface, 

 with a certain and variable number of goblet- and gland-cells. 

 The nuclei of the epidermal cells are oval or spindle-shaped. 

 These cells are attached to the subjacent tissues by a basement- 

 membrane of delicate fibrillaj which distinctly marks the 

 boundary between the epidermal epithelium and the subjacent 

 tissues. Beneath this epidermis is a chai-acfceristic granular 

 parenchyma (PI. XXXV. fii,^s. 8 & 9 ; PI. XXXVI. figs. 10 & 11 ; 

 PI. XXXVII. fig. 14; PI. XLI. fig. 33, par.), which contains a 

 great variety of elements, some being comi^arable to the 

 " Rundzellen " and " Langer'schen Blaseu " described by List 

 (27 fe) for the Mytilidas, some being dark-staining, apparently 

 glandular elements that open out between the epithelial cells 

 (fig. 33, gl.). As observed by List (27 b), this epithelium and the 

 underlying tissues are excessively variable in their characters. 



Over the surface of the muscle-attachment the epidermis is 

 difierent (PL XXXIII, figs. 4, 4ffi; PL XXXV. fig. 8, m.e]?.). 

 Here it consists of columnar cells, usually about 10-12 /j. long 

 and 2-4 /* broad, which pass over basally without any distinct 

 dividing-line into the muscle-fibres. Whether the transition 

 is direct, or whether in fact a connective-tissue junction is 

 present, cannot be determined from the available preparations 

 of Margaritifera vulgaris ; but in Myiilas ed/dis there is a 

 distinct connective-tissue layer (fig. 5, c.t.), the fibres of wliich, 



20* 



