466 ARTICULATES I WORMS. 



mainly very small or minute inhabitants of fresh water. 

 Daphnia, Cypris, and Limnadia are characteristic genera. 



The Limuloids have the abdomen reduced to a spine. 

 The Genus Limulus is represented by the Horse-shoe 

 Crab, Fig. 368, which attains the length of nearly two feet 

 in some cases.. This curious crustacean uses the same 

 organs both for walking and eating, the haunches of 

 the first six pairs of legs performing the functions of jaws. 



The Rotifers are minute and mainly microscopic crus- 

 taceans, varying from one sixteenth of a line to a line in 

 length, and the organs of locomotion are merely cilia ar- 

 ranged around the head. They are radiate in general 

 appearance, but crustacean in structure. 



SECTION III. 



THE CLASS OF WORMS. 



THE Class of Worms includes the lowest articulates, 

 those that present the typical structure of the branch in 

 the most simple and uniform manner. The body is long, 

 and composed of numerous similar rings or segments, 

 and the first, though scarcely differing from the others in 

 appearance, is the head. The nervous system is distrib- 

 uted equally throughout the whole length of the body, 

 and hence these animals are not destroyed when cut 

 asunder, as is the case in the higher animals, where there 

 is a great centre of the nervous system and nervous force. 

 When severed, worms not only do not immediately die, 

 but in many cases the head part at length produces a 

 tail, and the tail part a head, so that one individual in 

 this way becomes two. Division and self-repair, as above, 

 are in some a normal mode of multiplication. Worms 

 have been divided into three orders; Annelides; Ne- 

 matoids, including Gordiacei and Acanthocephala ; and 

 Trematods, including Leeches, Planariae, and Cestoids. 



