THE COCKLE. 55 
opens into the mantle cavity; it is called the ex- 
cretory, or non-glandular portion of the organ. The 
lower portion is of a rich brown colour, and opens 
into the pericardial space (see 8); this is called the 
secretory, or glandular portion. 
8. Circulatory system.—Above the organ of Bojanus is a 
pellucid space enclosed by a delicate membrane— 
the pericardium. Cut it open by a longitudinal 
incision. There will now be exposed to view the 
heart, consisting of a pale yellow oval ventricle, and 
two delicate, transparent, triangular auricles, one at 
each side, attached by the apex to the ventricle, and 
by the base to the branchial vein. The ventricle 
surrounds the posterior portion of the intestine, or 
rectum. The heart may be beating slowly. Count 
the pulsations. There are five or six in a minute. 
From the ventricle pass anterior and posterior aortas 
along the rectum. With a needle carefully detach 
these from the rectum. The anterior runs above the 
rectum, the posterior runs below the rectum, and 
passes between the pedal muscles. Lift up the 
ventricle with the forceps, slit it up, and remove it 
altogether. The rectum will now be seen to be quite 
unconnected with the heart. 
‘8. Nervous system consists of three principal pairs of 
ganglia :— 
The cerebral ganglia are united into one, which 
lies above the mouth, on the tendon of the 
anterior retractor of the foot, and in front 
of the protractor. It is yellow in colour. 
The parieto-splanchnic ganglia are also united ; 
they lie on the anterior surface of the pos- 
terior adductor muscle. They are white. 
The pedal ganglia lie close together in the foot, 
