THE TUNICATES. THE HEART AND VASCULAR SYSTEM. 



417 



275.) In the tunicates similarly arranged enteric pouches are formed, which 

 unite with the lateral walls of the body, or with infolded rudimentary appen- 

 dages, forming gill clefts that lead from the enteron into the atrial or peribranchial 

 chamber, and thence to the exterior. 



The Heart and Vascular System. — In both tunicates and cirripeds the 

 vascular system is imperfectly developed; nevertheless there are some interesting 

 points for comparison. In the tunicates there is a short, oval heart on the poster- 

 ior haemal surface of the thoracic region. It is similar in form and location to that 

 in primitive Crustacea, and the general arrangement of the associated blood chan- 

 nels is also similar. 



Figs. 285, 286. — Diagrams illustrating the manner in which a sessile, cirriped-like arthropod is supposed to give 

 rise to a tunicate. A-D, Seen from the side; E-H, same, seen from the neural surface. 



In the cirripeds the heart is absent. But in many of the primitive, short- 

 bodied Crustacea, a heart is present, and consists of a short oval sac containing 

 but one pair of openings (cladocera, Fig. 9, h.)., or a small number of them. 

 It no doubt arises in typical arthropod fashion from the fusion of the lateral meso- 

 derm plates of the posterior thoracic metameres. The circulation in the tunicates 

 is chiefly remarkable for the periodic reversal of the direction in which the blood 

 is made to flow, a condition generally assumed to occur nowhere else in the animal 

 kingdom. 



In the copepods a slowly pulsating heart, similar in appearance to that of 

 cladocera, may be present, but in parasitic forms it is said to be absent, although 

 there are certain channels through which the blood flows in a definite direction. 

 In Lepeophtheirus, for example, according to Scott, '■ the circulation, while wholly 



Liverpool Marine Biol. Com. Memoirs. VI, 1901. 

 27 



