300 CHORDATA. 
however, the atrium is not continued round the dorsal 
line of the pharynx. In Ascidia it was the mid-ventral 
portion which was incomplete. The atrium is continued 
backwards behind the pharynx and along the intestine 
until it terminates in the atriopore, through which the water 
has exit. 
Fig. 214.—-TRANSVERSE SECTION THROUGH AMPHIOXUS IN THE 
PHARYNGEAL REGION. 
(After LANKESTER, Boveri and others.) 
h 3) Dorsal Fin-skeleton. 
Nerve Cord. \ 
Notochord. 
_.Myomeric 
Muscle. 
be. 
ll 
Perivisceral 
Ceelom. 
.Pharynx leading 
into Atrial 
Cavity through 
Metapleural Cavity. - Clefts. 
Endostyle. Metapleural 
Ventral Blood-vessel. ‘old. 
The dark shading is the connective tissue and the light outside is the simple 
ectoderm. The myomeres and pharyngeal clefts are cut across as they run 
diagonally. The section is taken across C D in Fig. 213. 
Lying on the right side of the pharynx in the atrium is 
a long hollow sac, the “vez, which opens into the alimentary 
canal at the junction of pharynx and intestine. ‘The zz/es- 
tine is produced backwards as a long tube to the anus. 
The muscular system is well developed. The longitudinal 
muscles consist of a dorsal longitudinal system of muscles, 
aistoy called the myomeric muscles, extending through- 
‘out the length of the body. The muscle-fibres 
extend between numerous connective-tissue septa which are 
