156 
fourteenth sete they are either indistinct or absent. The 
epithelial lining of the stomach is strongly folded. It 
consists of columnar cells among which are numerous 
goblet-like cells, some of which produce a secretion which 
is probably digestive, while others form mucus (fig. 51). 
Commencing near the middle of the stomach, 2.e., about 
the level of the ninth sete, there is a well-marked ventral 
groove, the cells lining which are provided with long cilia 
which produce a current from before backwards (fig. 36). 
There are numerous smaller ciliated grooves on the lateral 
walls of the stomach and intestine in which the flow is 
downwards and backwards into the ventral groove. The 
ventral groove extends to the anus. ‘The intestine is 
lined by columnar cells, ciliated in the above mentioned 
grooves, among which only a few gland cells are present. 
Circular and longitudinal muscle fibres are present 
in the walls of the alimentary canal, but they are well 
marked only in the esophagus; in the stomach and intes- 
tine they are so feebly developed that these parts of the 
alimentary canal can have only shght powers of 
peristalsis. 
The process of digestion has not been fully investi- 
gated, but the series of events appears to be somewhat 
as follows: —During life the buccal mass and pharynx 
are constantly being everted and withdrawn, carrying 
sand into the esophagus. As the sand passes along the 
esophagus it is mixed with the mucus from the cells 
lining this part of the gut, and further back the secretion 
of the esophageal glands is poured upon it. The mixture 
then passes into the stomach, where the secretion from 
the mucus-forming and digestive cells is added to the 
mass. The swinging backwards and forwards of this part 
of the alimentary canal, brought about by the muscles of 
the body wall and by the protrusion and retraction of the 
