ANATOMY OF THE PEARL OYSTER. 67 



of the visceral ganglia : from the base of each a stout nerve (fig. 18, n.s.o.) passes 

 straight back, parallel with its neighbour and midway between the median line and 

 the margin of the adductor, till it reaches the pigmented pallia] sense organ of its 

 respective side a little anterior to the anus. 



After giving off the last-named nerves, the common pallial trunk passes backwards 

 and outwards, bifurcating almost immediately ; the external branch, the larger, is the 

 external pallial nerve and straightway bends outwards and passes into the mantle ; 

 the inner branch pursues a more median course, but in turn it soon divides. The 

 outer of the resultant nerves becomes the median pallial nerve ; the inner the 

 internal pallial nerve, the latter being the weakest of these three pallial trunks. By 

 the ramifying of these three nerves in the muscular and marginal regions of the 

 mantle, and by their anastomoses with a corresponding series of inner, outer, and 

 median branches given off by the anterior common pallial trunks from the cerebral 

 ganglion, a complex network of nerves termed the pallial plexus is formed. A some- 

 what similar arrangement of the pallial arteries is found the marginal pallial artery 

 having, like these pallial nerves, a double origin. 



In the case of the nerves, we find the external pallial, as indicated by the name, 

 passes directly to the margin along which it runs, branching as it goes ; the median 

 takes a parallel but more internal course, and anastomoses freely with its fellow 

 outside, while the third or inner branch passes forwards along the line of insertion of 

 the pallial retractors, branches being given off which meet and anastomose with 

 others from the median. On tracing these three nerves forwards we find that they 

 join the corresponding series arising from the anterior common pallial trunk. 



S ense - Organs. 



Specialized sense-organs are few and of low type in the pearl oyster, the only 

 structures that can come under this head being the otocysts, the osphradia, and the 

 pallial or abdominal organs of Thiele. The latter are a pair of slightly asymmetrical 

 laterally compressed tubercles lying upon the ventral surface of the adductor muscle, 

 one on either side, a little anterior to the anus. In each the long axis lies transverse 

 to the greater axis of the body. Dark pigment renders them conspicuous, and the 

 one to the right is distinctly the larger. It is also situated slightly further back than 

 its neighbour. In sections these sensory papillee are seen to be covered with epithelium 

 which contains specialized sense-cells. Plate IX., fig. 17a, shows the tip of one of 

 these sensory papillae. They are innervated by a special nerve from the visceral 

 ganglion, possibly derived from the cerebral, close to the posterior pallial nerve 

 (Plate VI., fig. 17, n.s.o.). The function of these organs is probably olfactory or of 

 such a (?) tactile nature as to test the quality of the water passing over the gills, or 

 to be stimulated by particles it contains. 



The otocyst, at the pedal mass, has numerous otoconia, and is supplied from the 



K 2 



