ANATOMY OF THE PEARL OYSTER. 75 



channel connecting nephridia ; Per.ar., pericardia] arch ; Inf. 3, rectum ; Ao.a., anterior aorta; 

 Ao.jp., posterior aorta. 

 Fig. 3. Dissection of dorsal surface of visceral mass, seen from above, enlarged ; to show branches of 

 anterior aorta (Ao.a.). 



4. Semi-diagrammatic vertical section through rectal ridge and adductor muscle, to show posterior 



aorta (An.ji.) branching into right and left posterior pallial arteries (nil. p. p.) ; enlarged. 



5. Course of the posterior pallial arteries from their origin in the posterior aorta close to the anal 



funnel (An./.) out to the pallial margin ; nat. size. 



6. Plan of the venous sinuses on the ventral and posterior aspects of the adductor muscle ; enlarged. 



For lettering see p. 70. 

 7. Plan of the same from the dorsal aspect, enlarged ; showing especially the main and branch 

 branchial afferent vessels (Br.aff.) to the gills. Rn.s., renal sinus. 

 8. Diagrammatic representation of the nephridia! system and its relations to the pericardium. 

 The arrows show the reno-pericardial canal (Bn.per.') and the external renal aperture (Hn.o.). 

 Enlarged view of the external renal aperture (lln.n.) and of the reno-pericardial opening 



(/;.//.'). 



Opening of the urino-genital vestibule at the junction of base of ctenidium (<1.) with the wall of 



the visceral mass (r.m.) ; enlarged. 

 Transverse section through the urino-genital opening, showing the emergence of the genital 



duct ; enlarged. 

 More enlarged view of the urino-genital opening; a., genital aperture. 

 Anastomosis of the muscle fibres of the pallial muscles. x 40. 

 A thin process from the growing margin of the shell, to show the " cellular " structure of the 



conchiolin in which the calcareous matter is laid down. x 40. 

 Margin of the shell showing the same structure, x 40. 

 Some prisms from a thick section of the prismatic layer. x 50. 



Part of a section of the prismatic layer decalcified to show the conchiolin framework. x 100. 

 Part of a section of the prismatic layer showing the ends of the prisms and the distribution of 

 pigment, x 50. A. shows one prism more magnified containing granular pigment. 

 19. A, B, C, stages in the decalcification of prisms, x 100; D, the conchiolin framework; E, part of 



conchiolin more highly magnified to show layers of deposition. 

 . 20. The nacre in surface view showing contour lines, x 300; A, more enlarged. 

 ,, 21. Another part showing the granular appearance sometimes seen. x 300. 

 ,, 22. Decalcified nacre, looking like a very much crumpled and folded membrane, x 300. 



PLATE VIII. 



1- ig. 1 . Section through the margin of decalcified shell to show the successively formed layers of prismatic 

 material separating at the free edge. x 50. 

 ., 2. Section of the mantle lobe showing the marginal and velar processes. x 50. 



A, B, O, D and E show the character of the epithelium, &c, at the points indicated, more 

 highly magnified, x 500. 

 ,, 3. Transverse section of the foot in the distal locomotor part. x 50. 

 4. region of the byssal groove. x 50. 



5. Transverse section of part of the byssus gland, showing the secretion lying in the lamellar ducts. 

 x50. 

 6. Dissection of posterior part of left ctenidium (the outer and inner gills of the left side). x 2. 



7. Diagrammatic transverse section across the two ctenidia, to show the median ciliated junction 



(M.cj.) between the two inner gills and the lateral ciliated junctions (L.c.j.) between the 

 outer gills and the mantle lobe, x 2. 



L 2 



