32 STUDIES IN BIOLOGY. 



The function of the plaited organs as respiratory organs helps to 

 explain the difference in their development in M. latus and in M. 

 edulis. In M. edulis the outer lamellae of the gills are free, and in 

 M. latus they are attached throughout. The result is that, in M. 

 edulis and also magellanicus , when the mussel is open under water the 

 angles between the mesosoma and gill, and between the gill and the 

 mantle, will be laved by water constantly renewed. In this case it 

 would be of advantage to the animal to have delicate tissues that 

 could function as respiratory organs : hence probably the great de- 

 velopment of the plaited organs in M. edulis and magellanicus. In 

 M. latus the outer lamellae are fastened down, and, although there is 

 doubtless a current of water through the gills, yet the facility given 

 for aeration of the blood will not be as great as in the two preceding 

 species, and the plaited organs are consequently small. It is not easy 

 to determine the primitive condition of these organs. If it is true 

 that they are always greatly developed when the outer gill-lamellae are 

 free, then, since this is the primitive condition of the gill, the state of 

 the plaited organs as we find them in M. latus would be secondary, 

 or a case of degeneration through loss of function. The bending of 

 the gill-lamellse at their base (Fig. 35) through the attachment of 

 these plaited organs is certainly a peculiar point, since if they were 

 more developed the descending lamella? would take the normal or 

 vertical position as in M. edulis. 



Note. — In explanation of Fig. 2, I may say that it represents the 

 gill-lamellse as cut off exactly along the line of attachment by the 

 rudimentary plaited organs. 



Aquiferous System. 



I have not been able to investigate this system. Sabatier states 

 that there is communication between the blood-system and the water, 

 the opening to the exterior being by a pore in the foot. 



Gills. (Figs. 35 to 40.) 



In the general structure of its gills M. latus resembles the other 

 species of Mytilus ; that is, its gills are two on each side ; each gill 

 consists of an ascending and a descending lamella, connected by bars 

 called interlamellar junctions. The gills are made up of filaments 

 placed one behind the other, and bent upon themselves to form the 

 two lamellae of each gill (Figs. 35 and 37). The filaments are linked 

 together by ciliated interfilamentar junctions. In M. edulis, as figured 

 by Peck (11), there are several interlamellar junctions in one filament, 

 which are placed one near the ventral side of the lamella, and the 



