THK DEVELOPMENT OF SYMBEANCHUS MARMOEATUS. 25 
tinuoiisintothe dorso-lateral pore-like openiiigsof the preceding 
stages, and is the functioning “exit’’ from the gill-cavity 
already described in Part I (compare PI. 8, fig. 19). Later 
on the lateral portions become obliterated and tlie single 
median ventral opening is the result. 
The alimentary canal of Symbranchus is straight, running 
from mouth to anus without bend or curve, showing little 
change in diameter, and very little to distinguish externally 
the different parts, at any rate behind the pharynx. But on 
dissecting open the canal of the fully adult Symbranchus, 
the surface appearance of the interior shows quite clearly 
oesophageal, stomach and intestinal areas. The sections at 
Stage 30 and onwards show that the stomach is of slightly 
wider calibre than the oesophagus, and that the characteristic 
macroscopic appearance is due to the presence of glands. 
Externally and microscopically it is possible to distinguish 
cardiac from pyloric part of the stomach, the latter having stout 
muscular walls. A pyloric valve, which, however, only begins 
to differentiate in Stage 32, connects the pylorus with the 
duodenum. The inner lining of the oesophagus and stomach 
is thrown into longitudinal folds, in sharp contrast to the 
intestine, which is honeycombed. 
The thymus at this stage is very conspicuous in both 
sagittal and transverse sections. The separate rudiments 
have all joined to form one large gland. 
In a median sagittal section the thyroid is distinguishable 
as a rectangular band of cells, stretching backwards from the 
root of the tongue to the first gill-arch. Transverse sections 
sliow that anteriorly the thyroid bifurcates. The structure 
at this stage is not glandular. The liver is like that of the 
adult, and the gall-bladder is walled in by two layers of flat 
epithelium. 
At Stage 32 the pyloric valve is differentiated. The pyloi ic 
part of the stomach projects back like a spout into the 
duodenum. Finger-like protuberances of the latter are 
packed all round the anterior part of this spout, thus hiding 
the constriction that would otherwise exist between the 
