Chap, v.] HEART OF CHORDATA. 193 



(that is, from its ventral towards its dorsal end) it 

 contains almost pure arterial blood, and may, there- 

 fore, be regarded as a systemic heart ; on the other 



/ 



hand, when the contractions are reversed in direc- 

 tion, the blood is nearly all impure, and, as it 

 largely passes to the gills, the heart may now be said 

 to be branchial or respiratory. 



The cardiac tube is sometimes constricted at 

 various points, but is never divided into distinct 

 chambers. The most remarkable condition is pre- 

 sented, not only among the Tunicata, but among all 

 known animals, by Appeiidicularia fureatn. In 

 it the heart consists of but two cells, which are con- 

 nected with one another by from twelve to twenty- 

 five processes, between which there are open spaces. 



In the Cephalochordafa we find an arrange- 



^^ 



ment which reminds us of what obtains in the 

 Annulata, inasmuch as there is no centralised con- 

 tractile heart, but the blood is only moved forwards 

 by the contractility of some of the great vessels. The 

 vessel of largest calibre is found in the neighbourhood 

 of the anterior gill- clefts; into this the blood passes 

 from the gills, and from it it goes into a vessel 

 which is connected with the right of the two so-called 

 aortic trunks. These two trunks unite with one 

 another behind the branchial area, and form a single 

 dorsal vessel, or " aorta," which extends backwards 

 along the body ; at the hinder end it is continuous 

 with a ventral vessel, which, on its way forwards to 

 the gills, gives off some branches to the rudimentary 

 liver (see page 161), and so forms a kind of rudimentary 

 portal system. (See page 206.) The blood from the liver 

 returns to the great ventral trunk, and, with the rest, 

 makes its way to the gills to receive a fresh supply of 

 oxygen. The branchial vessels form dilatations on 

 their course (bulbilli), and the contained blood either 

 makes its way directly into one of the aortas, or first 

 N 16 



