34 STUDIES IN BIOLOGY. 



exists in all specimens of the two last species that I have examined. 

 It is a space or cavity in the region of the posterior retractors, with an 

 opening near the hinder end of the mesosoma, and varies according 

 to the condition of the animal. Sabatier suggests that the existence 

 of the cavity is due to the great development of the posterior retractor 

 muscles : but there is no such cavity in M. latus, whose posterior 

 retractors are equally well developed, although differently arranged. 

 It may be owing to the posterior retractors of M. edulis and magel- 

 lanicus forming a single large bundle of muscle that a clear space is 

 necessary for their free action. 



There is one of these cavities in each side, and their openings are 

 shown in Fig. 3 posterior to the inner side of the genital papillae. 

 Their position is similar in M. magellanicus, but there is no such 

 opening or cavity in M. latus. 



Organ of Bo j anus. 



So far as the disposition of the main part of this organ is con- 

 cerned, there is little difference in latus, edulis, and magellanicus. It 

 is formed of two brownish masses stretching along the ventral side of 

 the body, one on each side of the mesosoma and foot, and opening 

 posteriorly by an aperture immediately behind the genital papilla of 

 each side (r a, Fig. 1). The ventral side of the organ in the anterior 

 half is in M. edulis and magellanicus continued into transverse bars or 

 ridges between the base of the gill and the foot, while in M. latus 

 there are no such ridges. The relations and function of these ridges 

 were discussed while considering the " organes godronnes." The 

 organ of Bo j anus functions as a renal or excretory organ, and it is 

 usual in Mollusca that these organs should have two openings, one to 

 the exterior usually closely connected with the genital aperture, and 

 one internal, into the ccelome or the pericardium representing it. 

 There is a great difference between M. edulis and latus with regard 

 to this internal opening. 



In M. edulis and magellanicus there are two openings in the 

 floor of the pericardium, one on each side in the anterior part (Fig. 

 34). From each of these a large oblique channel (called by Sabatier 

 the coidoir pericardique) passes round the anterior side of the posterior 

 retractor of its side to the organ of Bojanus (coul, Fig. 34). In the 

 posterior part of this " couloir " lies the oblique vein. Sabatier finds 

 by injecting the pericardium or the organ of Bojanus that this 

 " couloir " forms a direct and ready communication between the peri- 

 cardium and the organ of Bojanus ; hence it represents the internal 

 opening of the excretory organ into the ccelome. In M. latus the 



