30 STUDIES IN BIOLOGY. 



the horizontal vein in M. edulis. This sinus lies in the mantle just 

 below the level of the organ of Bojanus ■ and this organ and the 

 surrounding tissues are so lacunar that they act as a long blood-sinus 

 extending along the base of the gills. The blood from the foot, 

 mesosoma, muscles, and liver is collected in extensive intermuscular 

 ventral sinuses, and from them conveyed to the spongy tissues about 

 the organ of Bojanus : thus the whole blood of the body returns to 

 this neighbourhood. Some of it may pass directly into the longi- 

 tudinal vein, as I have found in front of the middle retractors small 

 channels leading up to the longitudinal vein like those above the 

 horizontal vein in M. edulis (Fig. 32), but the great part of the blood 

 must pass into the tissue of the organ of Bojanus, which surrounds 

 the longitudinal vein. The injection always enters the organs corre- 

 sponding to the " organes godronnes" of M. edulis (o gd, Figs. 2 and 3); 

 but these are so small in M. latus that they cannot greatly affect 

 the aeration of the blood. From the tissues of the organ of Bojanus 

 the blood will in part be transfused into the longitudinal vein, and 

 thence to the auricle. Part of the blood probably passes through the 

 gills in the natural course of the circulation, since there are efferent 

 branchial vessels, although the injection, possibly finding the course 

 into the longitudinal vein easier, refuses to enter the gills. But the 

 fact, noted by Sabatier, that a great part of the blood returns to the 

 heart without passing through the gills is very evident. As account- 

 ing for this, it may be that the blood exposed to the water on the 

 inner side of the mantle, and possibly also to water entering the 

 organ of Bojanus (although the function and action of this organ 

 are not altogether settled), is sufficiently aerated without the further 

 aeration obtained in passing through the gills. 



The investigation of the relations of the organ of Bojanus and 

 of the longitudinal vein is attended with great difficulties, for, while 

 in front of the entrance of the oblique vein the channel of the lon- 

 gitudinal vein is well defined, in the part behind the oblique vein the 

 organ of Bojanus, which is very irregular, projects into the channel 

 of the vein, and, as the tissues separating them are very delicate, in- 

 jection often proves a very unsatisfactory method of defining the 

 limits of the vein. 



Organes Godronnes, or Plaited Organs. 



For convenience I shall use the term "plaited organs," as a sub- 

 stitute for Sabatier's term, " organnes godronnes." These organs and 

 their position have already been referred to. They are transverse 

 lacunar bands of tissue communicating between the mantle and the 



