242 THE VITAL FUNCTIONS. 



Other ascidiae of the group. But what is still more 

 singular, these currents, after continuing in the same 

 direction for about a minute or two, gradually cease, 

 and are then renewed, but in an opposite direction ; 

 the blood flowing back in the same vessels in which 

 it had before been advancing : and these changes 

 succeed one another alternately.* The same phe- 

 nomenon has also been observed in the Salpa'\ and 

 the Pyrosoma :J and it has been ascribed to the 

 spiral contractions of the heart being made in diffe- 

 rent ways during the different periods, so that the 

 blood is propelled alternately into opposite trunks, 

 each vessel performing in turn the functions of an 

 artery and a vein. 



In the Aplysia, the large veins of the abdomen 

 exhibit numerous perforations, allowing of the pas- 

 sage of fluids from that cavity into those veins ; a 

 structure analogous to that of the dorsal vessel of 

 insects : but as they appear to perform only a func- 

 tion of absorption, they in no respect interrupt the 

 continuity of the vascular circuit ; for the fluid 

 which is thus absorbed is not again eff'used from the 

 vessels. Similar communications between the in- 

 terior of the venae cavae and the abdominal cavity 

 are also met with very generally in the Gastero- 

 poda and Cephalopoda. 



The circulating system of the Cephalopoda offers 

 several remarkable peculiarities. . The Sepia, in 

 whose highly organized system there is required 

 great additional power to propel the blood with 



* Lister; Philos. Trans. 1834,380. 

 f Van Hasselt, Ann. Sc. Nat. iii. 78. 



\ Milne Edwards ; Coniptes Rendus, x. 285 ; and Ann. Sc. Nat. 

 serie 2, xii. 375. 



