APLYSIA. 217 



respectively. The ventricle (Fig. 13, venfr.) is smaller than 

 the auricle and has thick, opaque muscular walls. There is a 

 constriction between auricle and ventricle, marked internally 

 by two crescentic flaps or valves. (Fig. 14.) Neither auricle 

 nor ventricle possesses an endothelium so that the muscle 

 strands are bathed directly by the blood. 



From the antero-ventral portion of the ventricle arises 

 the arterial trunk which supplies the whole body with blood. 

 It runs obliquely across the floor of the pericardium, and is 

 attached to the latter. Auricle and ventricle must be pushed 

 back to expose it. The sides of this aorta are sacculated, and 

 an internal examination shows that the sacculae are divided 

 off from the central cavity by fibrous strands. The function 

 of the sacculations, which form the crista aortae (Fig. 13, 

 cr. ao.), is not known. It is supposed by some to be a lymph 

 gland in which the leucocytes of the blood are manufactured : 

 by Grobben it was regarded as a gland for pericardial excretion 

 (Cf. Keber's Organ in Lamellibranchs.). Mazzarelli pointed 

 out that it is not alone in containing leucocytes in its wall, for 

 both auricular and ventricular walls produce them. 



On slitting up the crista aortae a semicircular valve is seen 

 guarding the entrance from the ventricle in order to prevent 

 regurgitation of blood into the ventricle. Three apertures or 

 paths of exit for the blood are visible : — 



1. An anterior opening lying ventral to the auricle. 

 This leads to the anterior aorta. 



2. A smaller aperture at the level of the ventricle and 

 leading to the gastro-oesophageal artery. In some Opistho- 

 branchs this artery arises as a branch from the posterior or 

 abdominal aorta. 



3. A posterior aperture leading to the abdominal aorfca. 



Arteries. 

 The arterial system shows great asymmetry. Three 

 arterial trunks arising from the heart are present, viz. : the 



