PHYSIOLOGY OF THE INVERTEBRATA. 205 



are, in some cases, if not always, contractile. The arteries 

 end in an extensively developed capillary system, but the 

 venous channels retain, to a greater or less extent, the charac- 

 ter of sinuses. The venous blood, on its way back to the 

 heart, is gathered into a large longitudinal sinus — ^the vena 

 cava — which lies on the posterior face of the body, close to 

 the anterior wall of the branchial chamber, and divides into 

 as many afferent branchial vessels as there are gills. Each 

 of these vessels traverses a chamber, which communicates 

 directly with the mantle cavity, and the wall of the vessel, 

 which comes into contact with the water in this chamber, is 

 sacculated and glandular." In Loligo media " the sacculated 

 afferent veins and branchial hearts contract about sixty 

 times a minute. The pulsations of these veins and of the 

 branchial hearts are not synchronous. The branchial veins 

 and the lamellae of the branchise also contract rhythmically," 

 but the branchial arteries do not contract. "The portion of 

 the branchial vein which lies between the base of the gill and 

 the systemic ventricle is very short, and it is hard to say 

 whether it contracts independently or not. Mechanical 

 irritation causes contraction both of the afferent branchial 

 veins and of the branchial hearts." (Huxley.) 



In Medone cirrhosus Professor Huxley has " observed regu- 

 lar rhythmical contractions of the vena cava itself, as well 

 as of its divisions, the sacculated afferent branchial veins, of 

 the branchial hearts, and of the branchio-cardiac vessels." 



The Tunicata. 



In the Ascidians the function of circulation differs entirely 

 from other Invertebrates. The peculiarity of this circulation 

 is the reversal at regular intervals of the direction of the 

 blood current. The heart is devoid of valves, and contracts 

 with a wave-like movement. If the wave is from below 

 upwards, "the blood passes into an abdominal vessel, thence 

 into transverse ascending canals that lead to the extraordinary 



