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 tree-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 " Tw T in-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 excavatious in the soft 

 pulpy flesh of the body, without any orifice for the dis- 

 charge of excrementitious matters. But there are other 

 kinds in which a much higher type of structure obtains ; 

 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 



