192 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



to rest on good foundation and is disproved by 

 a number of observations. The system of vessels 

 is better developed in the Cephalopoda than in other 

 Molluscs ; the definite arteries are more numerous, 

 and their finer ramifications are more distinctly capil- 

 lary in nature. The contractile power of the gills is, 

 no doubt, of some aid in the propulsion of the 

 blood ; the walls of the vessels connected with them 

 are, in the two-gilled Cephalopods, provided with 

 muscles, and the name of branchial heart has 

 been given to the enlarged portion of these arteries. 



The three great divisions of the Chord ates must 

 be dealt with separately. The Urochordata are 

 remarkable for an arrangement which, though not 

 unique in the animal kingdom (for it has been ob- 

 served also in the embryos of certain gastropods), is a 

 very striking characteristic, and most instructive pheno- 

 menon. It has been observed that in them the pul- 

 sations of the heart, having resulted in the movement 

 of the blood current in a forward direction, are, after 

 a pause, reversed, so that the blood flows backwards 

 instead of forwards. After this backward movement 

 has obtained for a time there is another pause, and 

 this is succeeded by a forward movement of the 

 blood. 



The heart of Tunicates has the shape of a tubular 

 or fusiform sac, and gives off a large vessel at either 

 end. In distinction to the forms already considered, 

 and in agreement with what obtains in the Verte- 

 brata, the heart appears to be an enlargement of a 

 ventral, and not of a dorsal, vessel. The trunks 

 which arise from it break up into vessels which, 

 according to the area of their distribution, may be 

 grouped as branchio-cardiac, cardio-splaiiclmic, or 

 splancho-branchial ; and, in addition to these, there 

 are a number of anastomosing vessels in the test. 

 When the heart contracts from behind forwards 



