PHYSIOLOGY OF THE INVERTEBRATA. 



communicate directly with the exterior by the organs of 

 Bojanns," 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 Gasteropoda and Oephalopack^ 

 have valves and columnse cam^ ; there are also contractile 

 expansions at the base of the vessels conducting the blood to 

 the branchiae. 



We iiow describe the circulatory system in three orders of 

 the MoUiosca. 



f 

 ^1 



t 



1 



Fig. 44.- 



y 



^A*- 

 W 



-^ i 



\if J 



i 



2 



-Diagram showing the Modifications of the Heart in 



THE INVERTEBKATA. 



1 = part of dorsal trunk of a worm. 2 = heart of Nautilus. 3 = heart of 

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



'u = ventricle. a = auricle. i = cephalic artery. c = abdominal artery. 



The arrows indicate the direction of the blood-current. 



(i) The Lamellihrancliiata. — The vascular system consists 

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

 vessels and sinuses. In Anodonta the heart lies ia 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 Mg. i8), and of two auricles 

 which are situated on each side of the ventricle. The 

 ventricle gives rise to the anterior and posterior aortse. 

 The auricles are muscular sacs, and communicate with the 

 ventricle by the auriculo-ventricular openings. These open- 



