174 BULLETIN OF THE BUREAU OF FISHERIES. 



the arteries to the mantle, returns, not through the kidneys or the gills, but directly 

 to the heart. 



The mantle and the gills constitute the chief respiratory organs, where the blood 

 is aerated. The significance of the mode of circulation is evident. The venous blood 

 returning from the body laden with waste products passes first to the kidney, thence 

 to the gills to be cleared of impurities and freshened with oxygen, after which it returns 

 to the heart in purified condition. The blood returning from the mantle requires no 

 further purification or oxygenation before entering the heart. 



Without a distinct brain, the body of the mussel is coordinated through a nervous 

 system, consisting of three pairs of nerve centers, which are connected together by 

 nerve cords. Two of these centers or ganglia lie one on each side of the gullet near the 

 mouth, a second pair is in the foot, while the third lies just beneath the posterior adductor 

 muscle. From these ganglia fine nerves are sent off to supply the various tissues 

 and organs. 



Though eyes and ears are not present, sensory organs are not entirely wanting. 

 A small organ near the ganglia beneath the posterior adductor is supposed to ser\'e to 

 test the purity of the water. Another, the otocyst, is sometimes found near the ganglia 

 in the foot and possibly serves as a balancing organ, by means of which the mussel 

 may feel whether it is in horizontal or vertical position. Sensory cells are found along 

 the border of the mantle, especially near the posterior openings for the passage of water. 

 (See p. 87.) 



The organs of reproduction comprise a large part of the body mass above the foot. 

 The ova or semen are discharged through small openings on each side of the body into 

 the chamber above the gills. In the case of the male the sperms are thence passed 

 out with the respiratory (exhalent) current and set free in the water. They may be 

 drawn into the female with the water of the inhalent current, to fertilize the ova perhaps 

 as they are passed down from the suprabranchial chamber into the tubes in the gills 

 where incubation takes place. In some species the reproductive tissue is brightly 

 colored — orange, pink, or red. 



STRUCTURE AND FUNCTIONS OF THE GILLS. 



The gills, as the name would suggest, are primarily breathing organs. Nevertheless, 

 they have an equal if not a greater function in food gathering, and, furthermore, in 

 fresh-water mussels and in some other lamellibranchs, the gills have acquired a third 

 office which is of coordinate importance with the other two. We have seen that the 

 incubation of the egg takes place in the water tubes of the gills, a part or all of which may 

 be filled with embryo mussels. The respiratory function of the gills of the female mussel 

 must be greatly reduced during the period of incubation, and this condition is made 

 possible by the fact that the mantle of the mussel plays an equal r61e with the gills 

 in respiration. In becoming adapted to this function of protection and perhaps nour- 

 ishment of the eggs and young, the gills of the female have undergone varied modifica- 

 tions in different species. In consequence, when gravid females can be examined, the 

 gills of different mussels are often found to be more strikingly distinct than is the external 

 form or any other obvious character. This is especially true when microscopic study 

 of the structure of the gills can be made. 



