VERTEBRATA ACRANIA. 143 



ated, and on into the dorsal arteries and dorsal aorta for general 

 distribution. There also appears to be an hepatic portal system 

 that is to say, the impure blood from the gut passes by portal 

 veins to the liver and enters a capillary system drained by hepatic 

 veins. 



The motive power by which circulation is effected appears to 

 be the contractility of the chief vascular trunks, especially of the 

 aortic arches, each of which commences in a small contractile 

 bulb which lies in the ventral fork of the corresponding primary 

 gill-bar. 



(2) Lymph System. This consists of a body-cavity or coelom, 

 and of other lymph-spaces. These are continuous with one 

 another and with the blood-system, but their exact relations are 

 complex and but ill understood. 



Coelom (Fig. 39). In a cross-section taken between anus and 

 atriopore this is easily recognised as a fairly wide space sur- 

 rounding the gut, but in front of this region the arrangement 

 is complicated by the large development of the atrial cavity. 

 Between atriopore and pharynx it is seen as a narrow space 

 surrounding the gut and continued round the liver. In the 

 pharyngeal region the most obvious parts of the coelom are the 

 dorsal cwlomic canals which run one on each side of the upper 

 part of the pharynx. The floors of these canals are obliquely 

 fluted to form suspensory folds connected with the primary gill- 

 arches, and each containing a ccelomic pouch running down the 

 outer side of its arch. 



A ventral ccelomic canal runs below the endostyle and receives 

 ccelomic tubes which traverse the chitinoid rods of the secondary 

 gill-arches. Besides this there are special sections of the coelom 

 .surrounding the gonads. 



The most important lymph-spaces, in addition to the coelom, are 

 (1) a metapleural canal running along each lateral fin, (2) spaces 

 round the fin-rays, (3) spaces in the myomeres of the head. 



6. Excretory Organs. It has been shown by experiment that 

 the organs which, under ordinary circumstances, excrete nitro- 

 genous waste, can also get rid of certain pigments artificially 

 introduced into the system. In this way a means is afforded of 

 recognizing such organs in doubtful cases. This method has 

 been employed for Amphioxus. The living animals were kept 

 in sea-water full of suspended carmine until they became pink in 



