170 BULLETIN OF THE BUREAU OF FISHERIES. 



all cases was to inject acid and basic dye substances and to observe their reactions in 

 the organ where they were eliminated. Indigo carmine, for example, is discharged 

 through the kidney and usually shows an acid reaction according to Cuenot ; Kowalevsky 

 and Letellier, on the other hand, state that it is usually neutral. Ammonium carminate 

 is eliminated through the pericardial glands where, all the authors agree, the reaction is 

 very acid. The strong reaction in this case was established by Letellier to be due to 

 the fact that the pericardial glands secrete hippuric acid in the free state. He concludes 

 that among certain lamellibranchs the urinary function is accomplished by means of 

 two separate organs, (1) the kidney which lies below the heart serving to eliminate 

 excess of water, urea, various nitrogenous bodies, phosphates, and possibly uric acid 

 and (2) the pericardial gland which covers the auricles serving to extract the acid con- 

 tained in the blood. This acid in at least two mollusks, Pecten and Cardium, was 

 found to be hippuric acid. Kowalevsky assumes that the kidney, or organ of Bojanus. 

 is analogous to the Malpighian corpuscles of vertebrates, which are neutral or basic in 

 reaction, and that the pericardial glands are analogous to the convoluted tubules, 

 which are acid. 



NERVOUS SYSTEM. 



The central nervous system consists of three pairs of ganglia, symmetrically placed 

 and connected by nerve commissures. The arrangement is strictly bilateral, one gan- 

 glion of each pair giving off nerves to its own particular side of the body. In Mytilus 

 edulis the form of the nervous system is but slightly modified from the typical type found 

 in the Lamellibranchia. 



The cerebral ganglia are triangular bodies which lie with their apices pointing back- 

 ward on the ventral side of the oesophagus just under the posterior edge of the lower lip 

 (fig- 159. CG, p. 171). They are laterally placed so that in an adult mussel they are 

 separated by a distance of 4 to 6 mm. In some specimens they are very conspicuous 

 to the naked eye, owing to the presence of an orange-red pigment, while in 

 others they are difficult to find, because the bright pigment is absent and their color 

 is like that of the tissue in which they lie. The cerebral ganglia are united by 

 the cerebral commissure which runs dorsally over the oesophagus (fig. 159, CC). Each 

 ganglion gives rise to a large anterior trunk, the anterior pallial nerve, and a posterior 

 trunk which contains the combined cerebrovisceral and cerebropedal nerves. Besides 

 these there are two fine fibers, one of which supplies the labial palps and the other 

 the eye. 



The anterior pallial nerve (fig. 159, APN) arises from the outer anterior corner of the 

 cerebral ganglion, runs forward and slightly outward over the anterior adductor muscle, 

 to which it supplies a few fine fibers, and then turning backward it traverses the inner 

 fold of the mantle edge, giving off along its course numerous side branches that form a 

 network of fibers in the border of the mantle. A short distance posterior to the anterior 

 adductor muscle the nerve trunk gives off a large branch (BA PA 1 ) that runs forward to 

 the ventral side of the anterior adductor muscle. 



The cerebropedal and cerebrovisceral connectives leave the posterior angle of the 

 cerebral ganglion in a common trunk that passes backward and outward across the 

 ventral side of the anterior retractor muscle. When it reaches the lateral side of the 

 muscle the trunk divides into its two separate components. The cerebropedal con- 



