126 
LIFE, IN ITS LOWER FORMS. 
tion, the Twin-worm was formed by the accidental union 
of two individuals, if abundant observation had not proved 
that this is the common mode of life belonging to the 
species. 
Each portion of the animal is complete in all its organs 
and economy ; possessing its own sets of suckers, its own 
mouth, its own digestive canal, with its trce-like ramifi- 
cations, its own perfect generative system, and its own 
elaborate series of vascular canals, — every organ or set of 
organs in the one-half finding its exact counterpart in the 
other. 
It scarcely detracts from the marvellous character as- 
sumed by this “ Twin-worm,” that, according to recent 
observations, the two halves have already enjoyed a phase 
of existence as distinct individuals. The organic union, 
or “ fusion ” of two such individuals, is necessary to the 
development of the generative system, which, up to that 
event, is wanting in each constituent half. 
All the intestinal Worms that we have been speaking 
of exhibit a low degree of organisation, and a very simple 
structure. No nervous system has been detected in them ; 
and the digestive canals are simple excavations in the soft 
pulpy flesh of the body, without any orifice for the dis- 
charge of excremcntitious matters. But there are other 
kinds in which a much higher type of structure obtains j 
the nervous and muscular systems are distinct ; the diges- 
tive canal is a tube, isolated in the midst of a visceral 
cavity with a proper outlet ; and the reproductive appa- 
ratus is distributed to distinct sexual individuals. 
To this tribe belong the various sorts of Thread- worms, 
not a few of which infest the human body. The common 
