110 



the pterocardiac and the zygocardiac ossicles the anterior 

 handle-like portion of the latter are also drawn down- 

 wards and inwards. Posteriorly the zygocardiac ossicles 

 are in contact with the exopyloric ossicles, which in their 

 turn articulate with the pyloric ossicle. Therefore if we 

 consider the zygocardiac and the exopyloric ossicles as a 

 single rod, we have a lever of the second order, the 

 fulcrum being at the anterior end and the weight in the 

 region of the zygocardiac tooth. Thus, the application 

 of the force at the anterior end rotates the tooth down- 

 wards and inwards, and the three sets of teeth meet in the 

 middle line. When the muscles relax the ossicles spring 

 back into their original position, partly because of the 

 elasticity of their joints, but mainly by means of the 

 action of the cardio-pyloric muscles. 



THE BLOOD VASCULAR SYSTEM 



(PI. VII, figs. 49, 50; Pis. VIII, IX). 



Briefly stated the scheme of circulation is as follows. 

 The pure blood returning from the gills passes into the 

 Pericardium by means of the Branchio-cardiac veins. 

 From the Pericardium the blood enters the heart through 

 the ostia. From the anterior end of the heart there 

 arise five arteries carrying the blood to the gonads, diges- 

 tive glands, fore-gut, and the front part of the body. 

 From the posterior region of the heart two median 

 arteries arise which supply the abdomen and the appen- 

 dages. The impure blood returning from the system does 

 not pass to the gills along definite vessels, but flows 

 through irregular spaces or sinuses between the various 

 organs. The blood frsm the sinuses eventually reaches 

 the gills and passes along the Afferent Branchial Sinuses 

 on the outside of the grills. The blood is distributed to 



