74 CALIFORNIA acade;my of sciences. 



eating that the fluid is under pressure, Indeed, I am in- 

 cHned to think that such is the case; and it appears to 

 me highly probable that to this is due the origin of this 

 method of heart movement. We know from the studies 

 of Lacaze-Duthiers, Roule, Herdman and others, on 

 various groups of Tunicates, that what, for the present 

 consideration at least, corresponds to a quite extensive 

 system of systemic capillaries exists in many species. 

 Besides, the area of surface of the containing vessels 

 within the branchial apparatus must be much greater than 

 that of the afferent vessels; consequently the friction 

 which the moving fluid must here overcome must be cor- 

 respondingly increased. 



So far as the retarding influence upon the blood of the 

 capillaries is concerned, we may then regard the blood of 

 Tunicates as having to be driven through two systems of 

 them, i. e. a branchial and a systemic, by the same im- 

 pulse of the heart in either direction. 



Now, it would seem that the collateral aids to the work 

 of the heart, which are made use of in various ways and 

 to varying degrees in most other animals in which a well- 

 formed blood circulatory system is found, are very poorly, 

 if at all, developed here. 



In the flrst place, the blood vessels, even where they 

 reach their highest development are scarcely more than 

 channels through the musculo-connective tissue. This 

 being the case, they can have very little of the elasticity 

 of vessels with true muscular walls ; and this fact, taken 

 together with the entire absence of valves, renders it quite 

 impossible thiit the vessels outside the heart should exer- 

 cise any power in propelling the blood through the capil- 

 lary S3^stem — properly here a lacunar system. 



Concerning the structure of the vessels as they exist 

 in the Cvnthiada?, which may be taken to represent as 



