77 



III order to inject tlie venous system the syringe can 

 be put into the alterent branchial vessel (fig. 16, Br. aff.). 

 Care must be taken that the tissue separating the two 

 liranchial vessels is not perforated. Further, when 

 injecting the venous system, the valves must not be 

 removed, but the convex valve should be broken away 

 carefully, piece by piece, with bone forceps, riglit uj) to 

 the attachment of the adductor muscle to the shell. The 

 muscle must not be separated from the valves, for a large 

 sinus will be otherwise broken into. If the oil mixture 

 is used for both these injections the course of many of 

 the main vessels can be followed, but it is not permanent 

 and will not allow of the dissection of arteries and veins 

 in the visceral mass and other deep-lying parts of the 

 body. For this latter purpose, ordinary table jelly 

 coloured with carmine, and melted with just a little 

 water, proved quite satisfactory. It is necessary, previous 

 to injecting with the hot jelly, to place the specimen in 

 \Aaini watei' for about half au hour. 



Arterial System (fig. 14). 



The blood leaves the ventricle by two main vessels, 

 the anterior and posterior aortae. 



The Anterior Aorta (figs. 14 and ID, Ao. mif.) is a 

 large artery which arises from the ventricle at the middle 

 of its dorso-anterior edge above the alimentary canal, and 

 passes forwards directly over the latter to the digestive 

 gland. There is an aortic dilation just after leaving the 

 ventricle, inside the pericardium, and this can be seen to 

 expand after each contraction of the ventricle. 



On reaching the posterior end of the digestive gland 

 the aorta passes dorsally along its middle line, giving off 

 a small vessel on each side (figs. 14 and 10, a.) which pass 



