THE COMMON MUSSELS. 39 



edulis (Fig. 18) in general disposition ; but I cannot make a detailed 

 comparison of their nervous system, as I had not time to dissect M. 

 edulis fully. The arrangement of the nerves from the pedal ganglia 

 seems to differ in M. latus and edulis ; but I have not the particulars 

 of their distribution in M. edulis (compare Figs. 17 and 18). 



Reproductive Organ. 



The general disposition of this organ is similar in the three species 

 we are dealing with. The greater part of this organ lies in the 

 mantle-lobes, and its ducts, branching throughout the mantle, converge 

 to a common stem near the anterior end of the pericardium, below 

 the retractors. From thence the ducts pass inward to the inner side 

 of the base of the gills, and proceed backwards, one on each side of 

 the mesosoma, between the ascending and descending lamellae of the 

 inner gill, opening in the genital papillae near the hinder end of the 

 mesosoma (g d and g p, Figs. 2 and 3) . Part of the genital organ 

 occupies the sides of the mesosoma, and the angles between the meso- 

 soma and the mantle-lobes. The animals are dioecious, but the 

 general plan of the organ and of its ducts is similar in both sexes. 

 The males have the mantle-lobes whitish-yellow ; the female are 

 usually reddish: this point was noted also by Lacaze-Duthiers (12). 

 The branches of the ducts in the mantle-lobes of M. latus seem to be 

 not quite as superficial as in M. edulis — at any rate they are so 

 obscure that it is difficult to define them ; whereas in New Zealand 

 specimens of M. edulis they are easily seen, although the ducts are 

 not as wide as represented by Lacaze-Duthiers in the European M. 

 edidis. The genital duct is readily traced on the ventral side of the 

 organ of Bojanus, especially if distended with the genital products. 



Are M. latus and edulis congeneric ? 



By reference to the following table 'and to the preceding pages it 

 will be found that, while in the general plan of structure M. latus 

 quite agrees with M. edulis and magellanicus, yet in the case of nearly 

 all important organs there are considerable differences. While M. 

 edidis and magellanicus agree so closely as to leave no question of 

 their being members of one genus, there is a very considerable interval 

 between them and M. latus. To pronounce definitely upon the 

 above question would require an intimate knowledge of a great 

 number of species of Mytilineae, when it is possible that all inter- 

 mediate stages between the above distinct forms might be found. My 

 study of the internal structure has extended only to the three species 

 mentioned in this paper, but, so far as this serves to decide, there 



