EXCRETORY SYSTEM. 419 



the system of lymph spaces and canals which lie below the cutis. In 

 the ventral poitions the septa disappear, and the enclosed spaces, 

 bounded by somatopleure and splanchnopleure, unite to form the 

 " splanchnoccel " which surrounds the gut. In the adult this space is 

 reduced anteriorly to small spaces and ccelomic canals, by the develop- 

 ment of the atrial chamber (see Figs. 178 and 179). This pushes the 

 somatopleure up before it as it develops, and is thus enlarged at the 

 expense of the true ccelom. The ccelomic spaces and canals contain 

 coagulable fluid, and represent the lymphatic system of higher forms. 



Besides the main trunk origin of the ccelom, there is an anterior 

 portion, which is separated off from the very front of the gut, and 

 is divided into two cavities, of which the right becomes large and 

 thin-walled, while the left becomes a small thin-walled sac, which has an 

 opening to the exterior. This may correspond to the head ccelom of 

 Balanoglossus, and to the bilobed head cavity which lies beneath the eyes 

 of fishes, and forms most of the eye muscles. 



Thirdly, there is a pair of pouches, which form the first pair of 

 muscle segments, and are continued out into the atrial folds. These 

 may correspond to the collar ccelom of Balanoglossus (MacBride). 



Circulatory system. — The blood is colourless, with a 

 few amoeboid cells. There is no definite heart, but the 

 branchial artery is rhythmically contractile. 



This branchial artery lies in the portion of the body cavity which 

 is enclosed by the endostyle, and is the anterior continuation of a large 

 hepatic vein from the ccecum. From the branchial artery a series of 

 smaller vessels arise, which pass up the primary gill-bars, and also 

 supply the tongue-bars. These unite on the dorsal surface of the 

 pharynx to form the right and left dorsal aortje, which join at the 

 hinder end of the pharynx to form a single vessel running backward 

 over the intestine, and breaking up into capillaries on its wall. From 

 the right dorsal aorta there arises a complex cf vessels supplying the 

 anterior region. From the capillaries of the intestine the blood is 

 collected in a sub-intestinal vein, which again breaks up in the 

 caecum. The cycle is completed by the capillaries which form the 

 hepatic vein. 



Excretory system. — A number of structures have been credited 

 with excretory functions. 



wall, which is cut through along its line of insertion. The result 

 is to show that the chamber is prolonged dorsally into a series of 

 bays (&.), which lie on the surface of the tongue-bars (t.b.). Into 

 these bays each of the nephridia (».) open.s by a pore (p.), while 

 they also open internally by many funnels (f.), fringed by very 

 large cilia (<:.). The bays are separated by ridges (d.) t formed by 

 a downgrowth of the walls of the ccelom over the primary bars 

 (p.b.). my. t a myotome ; sy., one of the synapticula connecting 

 the pharyngeal bars. 

 The lower figure is a more superficial view, to show the blood vessels 

 which form an anastomosing plexus (c.) over the walls of the 

 nephridia (nph.). d., Dorsal aorta ; cai., ccelomic space within 

 ' primary bar ; b.v. t blood vessel of secondary bar ; m., cut edge of 

 the wall of the atrial chamber ; other letters as before. 



