CIRCULATORY SYSTEM. 467 
segmented into a serjes 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 coelomic canals, by the 
development of the atrial chamber (see Figs. 249 and 250). The 
coelomic spaces and canals contain coagulable fluid, and represent the 
lymphatic system of higher forms. 
Besides the main trunk portion of the coelom, there is an anterior 
portion, which is separated off from the very front uf 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 coelom pouch may correspond to the 
head coelom 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 coelom about the 27th 
myotome. They are probably diverticula of the atrium. 
Circulatory system.—The blood is colourless, with a 
few amceboid 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 cecum. 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 (.), which lie on the surface of the tongue-bars (¢.4.). Into 
these bays each of the nephridia (z.) opens by a pore (0.), 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 ccelom over the 
primary bars (2.2.). y., Amyotome ; 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 (¢.) over the walls of the 
nephridia (#ph.). d., Dorsal aorta; ¢c@., ccelomic space within 
primary bar; 4.v., blood vessel of secondary bar; #., cut edge 
of the wall of the atrial chamber ; other letters as before. 
