They have gills within the casing, and that's how they get the oxygen.
(They; have; gills within the casing) (that; 's; how they get the oxygen) Context(that 's,List([39, 45))):(they; get; the oxygen)
that 's,List([39, 45))
They that they
have 's get
gills within the casing
how they get the oxygen
the oxygen