28 STUDIES IN BIOLOGY. 



connection is made between the hinder part of the horizontal vein 

 and the posterior part of the organ of Bojanus and longitudinal vein 

 by an obliquely-placed vein, the anastomosing vein {a v, Fig. 32). 

 This anastomosing vein receives on its course a vein from the sinus 

 among the fibres of the posterior adductor ; this sinus also sends a 

 vein to the horizontal vein to the front of the posterior adductor. 

 This double communication, Sabatier suggests, prevents the bursting 

 of the vessels through undue extension, since the blood, collecting 

 rapidly and failing to find a course in one way, by a reflux action 

 may avail itself of the other outlet. The blood from the mesosoma 

 in M. edulis returns by more or less lacunar passages to the region of 

 the organ of Bojanus and the longitudinal vein. There is a very 

 large venous sinus enclosed between the anterior retractors, which 

 collects the blood from the anterior part of the body; and there are 

 other haemal spaces among the bases of the muscles of the foot and 

 byssus. These intermuscular sinuses collect the blood returning from 

 the surrounding organs — the liver, the muscles, foot, &c. — also in 

 part from the mesosoma. The blood from these various sources 

 penetrates from the haemal spaces into the lacunar tissue of the 

 organ of Bojanus, and thence in part to the gills and in part 

 directly back to the heart by the longitudinal vein. The blood that 

 enters the gills traverses the gill-filaments, and is collected by the 

 efferent vessels of the gills which lie on the upper edge of the free or 

 ascending lamellae. The efferent vessels carry the blood forwards and 

 throw it into the anterior ends of the longitudinal veins between the 

 bases of the labial palps. To summarise the above, the venous blood 

 passes from the marginal sinus or from the ascending veins into 

 the horizontal vein, thence by small vessels into the " organes 

 godronnes," and thence, either directly or through the tissue of the 

 organ of Bojanus, into the longitudinal vein, and from that to the 

 heart. The other course is to interpose the gills between the organ 

 of Bojanus and the longitudinal vein, in which case the blood is con- 

 veyed to the anterior end of the longitudinal vein by the efferent 

 branchial vessels. The blood may also be collected by various sinuses 

 and thence introduced to the organ of Bojanus, to follow afterwards 

 either of the above courses. 



In M. latus the longitudinal vein has well-defined walls, and it 

 extends from near the anterior retractor impression to the posterior 

 adductor, widening as it goes backward (I v, Figs. 29 and 33) . It lies 

 just along the line of attachment of the descending lamellae of the 

 gills, and, when injected with colouring matter, is visible through the 

 bases of the gills when they are separated. 



