202 



PHYSIOLOGY OF THE INVERTEBRATA. 



communicate directly with the exterior by the organs of 

 Bojanus," or kidneys. The higher Mollusca have all of them 

 well-defined hearts, generally with auricles and ventricles 

 (Fig. 44), arteries and veins, though the capillary system is 

 still absent. The hearts of the GasUro'poda and Cc/phalopoda 

 have valves and colunmtc carnae ; there are also contractile 

 expansions at the base of the vessels conducting the blood to 

 the branchi^. 



We now describe the circulatory system in three orders of 

 the Mollusca. 



\ ^ J 



Fig. 44.— Diagram showing the Modifications of the Heart in 



the invertebrata. 



1 = part of dorsal trunk of a worm. 2 = heart oi A^antil//s. 3 = heart of 

 a Lamellibranch. 4 = heart of Octopus. 5 = heart of a Gasteropod. 



V = ventricle. a = auricle. /' = cephalic artery. c — abdominal artery. 



The arrows indicate the direction of the blood-current. 



(i) The Lamcllihranchinta. — The vascular system consists 

 of a heart, anterior and posterior aortae, and other blood- 

 vessels and sinuses. In Anodonta the heart lies in the 

 middle line of the body, and is surrounded by the pericardium 

 or blood sinus. It consists of a median ventricle, which is 

 perforated by the intestine (see Fig. 18), and of two auricles 

 which are situated on each side of the ventricle. The 

 ventricle gives rise to the anterior and posterior aortae. 

 The auricles are muscular sacs, and communicate with the 

 ventricle by the auriculo-ventricular openings. These open- 



