ig2 THE ASCIDIANS. 



which traverse the wall of the branchial sac in every 

 direction. 



In the second chapter it was pointed out that the 

 Vertebrate heart arose as a specialisation of a portion of 

 the primitive sub-intestinal blood-vessel whose calibre was 

 originally uniform throughout, and that in Amphioxus the 

 cardiac region of the vascular system retains its primitive 

 tubular character. 



Very different is the actual origin of the Ascidian heart ; 

 although it is simply a dilated tubular structure, yet it 

 arises entirely independently of and prior to the rest of 

 the vascular system at a time, in fact, before the formation 

 of the muscular mantle and before the atrial cavity has so 

 far extended itself as to almost entirely replace the original 

 body-cavity. The blood-sinuses of the Ascidians are rem- 

 nants of the latter. 



With the formation and growth of the atrial cavity, the 

 perforation of the stigmata, and the development of the 

 muscular mantle, the original body-cavity becomes reduced 

 to a system of narrow canal-like spaces which constitute 

 the above-mentioned blood-sinuses. The general distribu- 

 tion of the blood-sinuses can be made out from Fig. 95. 

 There are two main longitudinal sinuses, one below the 

 endostyle and another above the dorsal lamina, while 

 others are scattered irregularly in the muscular mantle ; 

 others again lie in amongst the viscera forming the inter- 

 spaces between the various parts ; and finally the bran- 

 chial bars between the stigmata are all hollow, and their 

 cavities are placed in communication with the system of 

 sinuses at intervals as shown in Fig. 95. 



The periodic contraction of the heart of Ascidians takes 

 place on a highly characteristic and unique plan. Each 

 systole occurs as a peristaltic wave of contraction passing 



