THE COMMON MUSSELS. 41 



Mytilus edulis — continued. 



Muscles. An anterior adductor present. 



Anterior retractor usually attached to the foot. 

 Posterior retractors of the byssus united in one bundle. 

 Cavite des fiancs. Of considerable size. 

 Organes godronnes. Large. 

 Alimentary canal. Pyloric caecum forming one tube "with first 



part of intestine, not extending past posterior adductor. 

 Circulatory system. Vascular system double and equilateral. 

 Mantle wholly supplied from aortic bulb or anterior aorta. 

 Marginal artery absent. 

 Posterior aorta small. 

 Two anterior gastro-intestinal arteries, and two posterior, in 



addition to the posterior aorta. 

 Venous system well defined. 



Oblique vein anterior to the whole of the posterior retractors. 

 A marginal sinus present. 

 Organ of Bojanus. Communicates freely with the pericardium 



by a large couloir or passage. 

 Gills. Ascending lamellae quite free at their upper edge. 



Interlamellar junctions more numerous, and several placed 

 together at the middle. 

 Mantle. Mantle-lobes joined throughout a great part of the 

 posterior end. 



Methods of Research. 



A great part of the structure, as for instance the structure of the 

 various parts of the alimentary canal, nervous system, &c, has of 

 course been made out by the ordinary process of dissection, but with 

 the vascular system it is necessary to have recourse to injection. 

 The most convenient apparatus for injecting the mussel is a small 

 injection-syringe provided with very fine nozzles made of glass tubing, 

 these being attached by short lengths of indiarubber tubing. I have 

 employed several materials for injection : (1.) As recommended by 

 Sabatier (7), lard thinned with turpentine and coloured with ver- 

 milion or blue. This answers well enough for injecting, but the oil- 

 bubbles that rise while dissecting under water are inconvenient. 

 (2.) Plaster-of-Paris coloured red or blue. This is the most con- 

 venient, as it sets and maintains the form of the vessels while dissect- 

 ing. (3.) A strong solution of gum-arabic coloured either with 

 vermilion, carmine, or the prussian-blue precipitate obtained from 

 salts of iron. This has one point of advantage over the plaster-of- 



