G26 PISCES. 



(fig. 309, m m), which are little contractile cavities situated at the com- 

 mencement of each branchial vessel, forming so many little hearts acces- 

 sory to the preceding. Their number varies with that of the branchial 

 arches, from five-and-twenty to fifty on each side, their office obviously 

 being to distribute unrespired blood through the branchial apparatus. 

 No branchial veins can be distinguished in the living animal ; but by 

 carefully detaching the branchial chamber and laying it on a strip of 

 glass, it becomes apparent that the aorta, situated upon the dorsal aspect 

 of the respiratory cavity, receives the veins supplied from each branchial 

 arch. 



(1680.) Aortic arch performing the functions of a heart. The blood 

 of the Amphioxus is not, as in fishes, entirely supplied to the aorta 

 through the medium of the branchiae, but is partly conveyed immediately 

 into that vessel through two large trunks, the representatives of the 

 ductus arteriosus (fig. 309, fc), which directly unite the median arterial 

 heart with the aorta, and are, to a certain extent, continuations of the 

 heart itself. They are, however, themselves contractile organs, and are 

 actively employed in the propulsion of the blood, as is the aorta itself 

 (fig. 309, i), which, doubtless, performs the functions of a heart. 



(1681.) The heart of the vena portce (fig. 309, o) is a long vessel, 

 which runs along the under surface of the intestine as far as the hepatic 

 caecum; its contractions are readily observable in the living animal, 

 the intervals between each being exactly the same as in the other hearts 

 above mentioned. 



(1682.) The heart of the vena cava (fig. 309, n) is placed opposite to 

 the preceding that is to say, on the dorsal aspect of the hepatic caecum ; 

 it is at first of small size, but gradually becoming larger, ultimately 

 empties itself into the arterial heart (I), which it supplies with blood. 



(1683.) The contractions of the vessels, or hearts, above described 

 succeed each other in such a manner that each in turn becomes gradu- 

 ally filled, while others contract. The systole of the arterial heart does 

 not commence before the act of contraction has been completed in all 

 the rest of the system. Moreover each trunk contracts in succession 

 with so much energy that it seems to empty itself entirely, and remains 

 for some little time un distinguishable, from which circumstance it neces- 

 sarily results that any given portion of the blood will have passed 

 through the entire round of the circulation in the time which elapses 

 between the consecutive contractions of the same portion of the vascular 

 system a space of time which observation shows to be in Branchiostoma 

 about a minute. 



(1684.) But, perhaps, the most remarkable feature in the anatomy of 

 this singularly- organized being is the apparently complete absence of a 

 brain. The medulla spinalis, slightly thickened towards the central 

 part of the body, tapers off posteriorly as it approaches the tail, where it 

 terminates in a point ; and towards the anterior part of the body, as 



