CLASS VERMES. 9 



external hardened cases, as in the beetle and other 

 insect tribes; nor yet a calcareous sheU, as in the 

 univalve or bivalve moUusks, such as the snail 

 and oyster. With few exceptions, their bodies are 

 unprotected by any kind of hardened envelope, and 

 they possess no articulated members of locomotion. 

 They are usually enclosed in a tough skin, and their 

 bodies are composed of a series of segments, all of 

 which, with the exception of the first, which repre- 

 sents the head, resemble each other in almost every 

 respect. 



Some of the most recent and accurate writers on 

 zoology have divided the whole class into four orders, 

 possessing distinct and well-marked peculiarities*. 



1. The lowest of these is that of the Nematoid or 

 round worms, whose bodies are not annulated, nor 

 divided into distinct sections, but present an almost 

 unbroken cylindrical form. These are for the most 

 part intestinal worms ; and the Ascaris and Strongylus, 

 the former inhabiting the intestines of man, the 

 latter an enormous, usually fatal worm found in the 

 kidney of the pig, may serve as examples of the 

 order. 



3. The next division, that of the Platyelmia, or 

 flat worms, is more numerous than the last. In some 

 cases, as in the Cestoid worms (e. g. the Tarda, or 



* Vog^, Zoologisclie Briefe. Frankfort, 1851. — Carpenter, 

 The Microscope. London, 1856. — See also Burmeister's Zoolo- 

 gical Atlas. Berlin, 1843. — Achille Richard Elements d'Hi- 

 stoire natureUe. Paris, 1849. 



B 5 



