204 president's addkess. 



interesting internal pests — Entozoa. The creatures known in 

 their adult condition as tape-worms are in all cases deri\'ed from 

 the flesh of some animal other than that in which they attain their 

 mature form, so that to complete the life-cycle of any one of these 

 creatures, successive residence is necessary in the bodies of two 

 distinct species of animal, which are thus called the "interme- 

 diate host" and the " final host." And the worm passes through 

 two marked changes of form, — a " cystic" or bladder-like form in 

 the first or " intermediate" host, a " cestoid" or flattened, tape- 

 like form in the final host. The tape-worms which in the adult 

 form infest the human animal in our country are chiefly two, 

 Taenia solium, which is derived from the flesh of the pig, and 

 Tcbnia mediocanellata, from that of the ox. Other rare forms 

 may be left out of the account. In the mature, tape-like form 

 the animal consists of a vast number of segments, each of which 

 contains a complete armamentarium of reproductive organs, and 

 an opening or pore through which the fertilized ova are thrown 

 into the digestive canal of the host. These are produced in 

 enormous numbers, but are incapable of further development 

 •except in the internal economy of some other animal. Taking 

 the case of Tmnia solium, some of these multitudinous ova must, 

 in order to develop, find their way into the alimentary tract of 

 the pig. The changes which then happen are these ; the hard 

 and resistent external wall of the egg becomes softened or dis- 

 solved, the contained embryo is set free, and develops a set of 

 sharp spicular or needle-like appendages, by means of which it 

 is enabled to perforate the coats of the intestine, thus finding its 

 way into a blood-vessel. In this way great numbers of ova may 

 be conveyed into the blood circulation : through the coats of the 

 blood-vessels they again bore their way into the flesh of the pig, 

 where they enter upon the "cystic" stage of existence. In this 

 stage there is developed a head, armed with suckers and a crown 

 of booklets, but without a mouth, the whole nourishment of the 

 animal throughout life being carried on by simple absorption 

 through the general body suiiace. Behind the head there is a 

 large vesicle or cyst, the whole being ensconced amongst the 

 muscular fibres of the host. In this condition tlic creature 



