HEART OF VEBTEBKATES. 



63 



a blood sinus or as a system of blood-lacunae; or the arteries and 

 veins are connected by a network of delicate vessels, the capillaries. 

 If the connection between arteries and veins is effected by capillaries 

 in all parts of the vascular system, and the body cavity, as in the 

 Vertebrata, no longer functions as a blood sinus, the vascular system 

 is spoken of as being completely closed. 



In the Vertebrates and segmented worms the vascular system ob- 

 tains a considerable development before a true heart is differentiated 

 in it. At first rhythmically pulsating sections, very frequently the 



FIG. 55. Nervous system and circulatory organs of Paludina vivipara (after Leydig). F. 

 tentacle ; Oe, oesophagus ; Cg, cerebral ganglion with eye ; Pg, pedal ganglion with 

 adjacent otocyst ; Vg, visceral ganglion ; Phg, pharyngeal ganglion ; A, auricle of 

 heart ; Ve, ventricle ; Aa, abdominal aorta ; Ac, cephalic aorta ; V, vein ; Vc, afferent 

 vessel. r, gill. 



dorsal vessel, or the lateral vessels connecting this with the ventral 

 vessel (fig. 56), serve for the propulsion of the blood. 



Similarly amongst the Vertebrata, the lancelet (Amphioxus) 

 possesses no distinctly differentiated muscular heart, the function of 

 that organ being discharged by various parts of the vascular system 

 which are contractile. The arrangement of the vessels supplying 

 the pharyngeal section of the alimentary tract, which has a respiratory 

 function and is known as the branchial sac, admits of a comparison 

 with the vascular arrangement of the segmented worms, and repre- 

 sents the simplest form of the vertebrate vascular system. The 

 longitudinal vessel which runs in the ventral wall of the branchial 

 sac gives off numerous lateral branches, which ascend in the branchial 

 walls. These lateral vessels are contractile at tkeir point of origin 



