REPORT ON THE TUNICATA. 
21 
The Alimentary Canal. 
This tube, consisting of the oesophagus, the stomach, the intestine, and the rectum, 
varies greatly in its size, its arrangement, and its position in the body in the different 
groups of Compound Ascidians, In the BotryUidse it lies almost entirely alongside the 
branchial sac, whde in most other forms it extends for a longer or shorter distance behind 
it. The typical and probably the primitive arrangement is that shown in fig. 4, A., where 
the oesophagus {oe.) commences at the posterior end of the dorsal edge of the branchial 
sac {hr.s.) and runs backwards to open into the anterior end of the ovate stomach [st.). 
Fig. 4. — Three diagrams showing some of the more important variations of the alimentary canal found in 
Compound Ascidians. A. is prohahly the jirimitive condition, and corresponds to Ciona amongst Simple 
Ascidians. In C. an arrangement is shoivn, corresponding to Ascidia, which has been jiroduced by 
pulling up the intestinal loop or by a downward extension of the branchial sac. B. shows a twisting of 
the loop, which causes the stomach to lie on the dorsal edge of the abdomen. B. may very readily be 
converted into C. 
a., anus; A atrial aperture; Br., branchial aperture; Jr. s., branchial sac; i., intestine; m., mantle; cc., oesophagus ; 
r., rectum ; st., stomach. 
The intestine {i.) arises from the posterior extremity of the stomach and runs backwards 
for a short distance, and turns dorsally and then anteriorly to form a narrow loop. 
It may then be called the rectum (r.), which runs forwards alongside the stomach and 
oesophagus till it reaches the neighbourhood of the branchial sac, where it ends by 
opening {a.) into the peribranchial cavity in the neighbourhood of the atrial siphon [At.). 
From this primitive form two series of modifications have started. In the one the 
intestinal loop has been drawn forwards so as to lie upon the left side of the branchial 
sac with the intestine in front of the stomach. This produces the state of affairs found 
