VERMES. 69 



which is known as Trichina spiralis (fig. 51) ; a series of models are 

 shown which give a good idea of the structure of the female and 

 the smaller male. The young make their way through the walls 

 of the stomach of their host, and encyst themselves among its 

 muscles : a piece of a sternothyroid muscle is shown, taken from a 

 man in whose body it is calculated there were forty millions of 

 encysted Trichina. 



Other Nematodes infesting man, such as Filaria sanguinis 

 hominis, are too small for exhibition. 



Plants are not free from the attacks of Nematodes, and examples 

 are shown, accompanied by an illustrating figure, of the Ear-cockle 

 gall of wheat, the gall being due to the injuries inflicted by a 

 minute Thread-worm — Tylenchus tritici. Wheat is, of course, by 

 no means the only cultivated plant that is attacked by these minute 

 worms; the history of most has, however, still to be made out. 



Holding a somewhat uncertain position in relation to the Round- 

 worms are the parasitic Acanthocephali (Thorn-headed Worms) 

 and the free-swimming Chcetognatha, or Bristle-jawed Worms ; 

 examples of both of these groups are shown, together with diagrams 

 illustrative of their general structure. 



Axnulata. — The creatures that are most familiarly called worms 

 are to be found in Case III. ; here are a few examples of the nume- 

 rous kinds of worms that are found living freely in the sea, of 

 earth- and freshwater Worms, and of the Leeches. All these 

 worms are distinctly characterized by the fact that they consist of 

 a number of definite rings (somites), whence they have been called 

 Annulata. The marine Worm and the Earthworm differ from the 

 Leech in that these rings are provided with setae or bristles, of 

 which there are a number in each bundle in the marine, and a few 

 only in the terrestrial or freshwater form : hence the marine Worms 

 are called Polychceta and the latter Oliguclueta. 



The former are divisible into two great groups. There are those 

 that are free-swimming and are able to forage for themselves, such 

 as the lovely Sea-mouse {Aphrodite aculeata), the large Eunice 

 gigantea, the common Nereis pelagica, or the exquisitely coloured 

 Chloeia flava. Others live a more retired life, dwelling in tubes, 

 which they fashion for themselves ; they lead either a solitary or 

 a social life. Here we have examples of Sabella, Sabeilaria, Ser- 



