ANATOMY OF THE PEARL OYSTER. 65 



visceral mass, and below it fuses with the body-wall and so forms part of the' roof of 

 the adductor embayment of the supra-branchial chamber. 



The reno-pericardial tubes (Rn.per., Plate VII., fig. 8) are a pair of wide lateral 

 prolongations of the pre-cardiac part of the pericardium, thin-walled and mem- 

 branous, and directed forwards. Each gradually narrows towards the anterior end, 

 where it opens into the non-glandular part of the nephridium of its own side. The 

 aperture is a curved slit, with the concavity facing towards the ventral aspect 

 (Rn.per/). It has but one lip, the tube opening at a very acute angle. It is 

 situated upon the inner wall of the nephridium, immediately to the rear of the 

 external renal aperture. Usually, but not invariably, a small area around is rendered 

 conspicuous by flecks of brown pigment. 



Compared with the total bulk of the body, the size of the nephridial system is 

 small, especially when we bear in mind the comparatively great size of this organ in 

 some other Lamellibranchs, such as Anodonta and Cardiurn. There are, however, 

 also the accessory pericardial glands, described by Grobben,* possessing an excretory 

 function, situated on the walls of the auricles and on the neighbouring part of the 

 pericardial wall; and it is the dark-brown colouring of these glands which renders 

 the auricles most conspicuous objects in the dissection of the pearl oyster (see 

 Plate VI., fig. I). These glandular outgrowths increase largely the secretory area, 

 as the auricular walls are thrown into numerous pouches, which are of a spongy 

 st lucture, with deep folds of the inner surface dipping down into the blood stream. 

 The epithelium shows large cubical, rounded or ovate cells packed with concretions 

 ami granules (Plate IX., fig. 13). The lower or auricular end of the pericardium is 

 also glandular, and has its epithelium thrown into folds formed of granular vacuolated 

 cells (Plate IX., fig. 14) of the same character as those of the nephridium. The 

 secretion from all these pericardial glands passes by the wide reno-pericardial ducts 

 into the nephridia, and thence gains the exterior by the renal aperture. 



THE NERVOUS SYSTEM AND SENSE ORGANS. 



The nervous system of the pearl oyster is of the ordinary Lamellibranch character, 

 and is very similar to that of Mytilus edtdis, the common mussel. The bi-laterally 

 symmetrical central nervous system has 3 pairs of ganglia : (l) the cerebral ganglia 

 at the sides of the oesophagus (Plate VI., fig. 15, Cer.g.), (2) the pedals conjoined to 

 form a single ganglion (fig. 16, Ped.g.) at the base of the foot, and (3) a pair of large 

 visceral or parieto-splanchnic ganglia (fig. 15, Par.sp.g.) lying upon the anterior 

 surface of the adductor. These are connected as follows : 



Stout paired nerves, the cerebro-visceral connectives (C.v.con.), link the cerebral 

 with the parieto-splanchnic ganglia (fig. 15), while a pair of similar cords cerebro- 

 pedal connectives (C.p.con.) joins the cerebral with the pedal nerve mass (fig. 16). 



* 'Arbeit. Zool Instit. Wien,' Bd. VII, 1888. 

 K 



