WORM'LIKE AND LEECH-LIKE ANIMALS 



Chapter 5 



When Linnaeus, in the eighteenth century, was developing the system of 

 classification upon which our modern system is based, he grouped a large 

 number of soft-bodied invertebrates into one division which he called Vermes 

 or Worms. This tendency to call any creeping invertebrate a worm still per- 

 sists among the general public, and caterpillars are called cabbage worms or 

 measuring worms even by people who know them to be insect larvae. As 

 scientific knowledge increased, more and more animals were taken out of 

 Linnaeus' group of Vermes. At the present time the naturalist acknowledges 

 three main groups of land and fresh-water worms, which constitute three phyla 

 in our modern system. These are the Platyhelmmthes or Flatworms, the T^e- 

 mathelminthes or Round Worms and the Annelida or Coelhelminthes, 

 the Segmented Worms. Another small group, the J^emertea, are sometimes 

 placed with the Platyhelmmthes. The Rotifera also are sometimes grouped 

 as Vermes, but are here treated in a separate chapter. 



Some other forms in our ponds look very much like worms, and the ama- 

 teur is often deceived by them. Several of the insect larvae, especially the 

 midges, have a strong resemblance to the bristle worms or aquatic annelids, 

 but may be distinguished by their distinct heads. On land, the slugs are often 

 mistaken for worms. These are in reality shell-less mollusks, distinguished by 

 two pairs of tentacles and by the mantle which covers part or all of the back. 



The flatworms are best known for the more disreputable of their number, 

 the parasitic flukes or tapeworms. Most North American free-living forms 

 are small and inconspicuous, seldom becoming more than an inch or two in 

 length. They are finely ciliated animals, called Turhellaria from the currents 

 which their cilia set up in cloudy water. The cilia themselves are not apparent, 

 but the animals appear to move by a steady, effortless, gliding motion. When- 

 ever pond weeds are brought in for the aquarium or for the study of their 

 animal guests, some Turhellaria are likely to be found. An investigation of the 

 under sides of submerged rocks or logs is also likely to reveal some of these 

 tiny, flattened creatures. The largest ones might be mistaken for leeches, but 

 they do not have the posterior adhesive discs possessed by leeches. A few live 

 in moist places on land and may sometimes be found in greenhouses or under 

 boards in damp corners of gardens. 



One of the peculiarities of the Turhellaria is the position of the mouth, 

 which is situated in the middle of the ventral surface. From the mouth a 



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