CIRCULA TOR Y S YSTEM. 467 



segmented into a series of small sacs. These lie one behind the other, 

 and lose all connection with the gut. Each ultimately divides into two 

 a dorsal thick-walled portion, and a ventral thin-walled portion. 

 The dorsal portions form the body musculature, and retain their seg- 

 mentation. Their cavity, the myoccel, persists to some extent in the 

 adult, forming the system of lymph spaces and canals which lie below 

 the cutis. In the ventral portions the septa disappear, and the enclosed 

 spaces, bounded by somatic mesoderm and splanchnic mesoderm, 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 

 development of the atrial chamber (see Figs. 249 and 250). The 

 ccelomic spaces and canals contain coagulable fluid, and represent the 

 lymphatic system of higher forms. 



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

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

 divided into two cavities. The right becomes the cavity of the snout 

 in the larva, but is almost obliterated in the adult. The left becomes 

 the pre-oral pit. This anterior ccelom pouch 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). 



Two brown canals or atrio-ccelomic funnels discovered by Professor 

 E. Ray Lankester open into the dorsal part of the atrium about the 

 level of the junction between pharynx and intestine, while their 

 anterior ends project into the dorso-pharyngeal ccelom about the 27th 

 myotome. They are probably diverticula of the atrium. 



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 caecum. 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 



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 (3.), which lie on the surface of the tongue-bars (/.3.). Into 

 these bays each of the nephridia (.) opens' by a pore (<?.), while 

 they also project internally by blind funnels (/.), fringed by very 

 large solenocytes (c.). The bays are separated by ridges (d.\ 

 formed by a downgrowth of the walls of the coelom over the 

 primary bars (/.3.). my., 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 (npk.). d., Dorsal aorta ; cce., coelomic space within 

 primary bar; b.v., blood vessel of secondary bar ; m., cut edge 

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



