CLASS VI. ANNELIDA. 



615 



he has seen the revival of one of these Uttle creatures, on being put in water, after having been 

 four years thus completely desiccated ! These animals comprise two orders, the Natantia and 

 the Sessilia. 



ORDER 1. NATANTIA. 



This term means sioimming, and is applied to those species of the class which are free and 

 swim about from place to place. These have a sucker-like organ, or a small pair of forceps, at 

 the tail, by which they are enabled to fix themselves when they wish to set their wheels in mo- 

 tion, in order to obtain food. They are divided into two families, the Polytrocha and the Zygo- 

 trocha. 



ORDER 2. SESSILIA. 



This term means sitting, and is applied to those species of the class which remain fixed and 

 stationary. They include two families, the Floscularidce and the Megalotrochidce. 



THE LOB-WORM. 



Clas§ VI. AWr¥Er.II>A. 



In the Annelida, which include the Red-blooded Worms, the division of the body into seg- 

 ments is usually distinctly recognizable ; the majority live in water, or in damp situations ; a very 

 few only ai-e parasitic in their habits. In most the head is distinctly marked, and furnished with 

 organs of sense, such as eyes, tentacles, and in some instances auditory vesicles. Tlie nervous 

 system, in the higher forms, exhibits the articulate type of structure very distinctly ; it usually 

 consists of a series of ganglia running along the ventral portion. Tlie digestive apparatus con- 

 sists of a straight intestine, running through the body from one extremity to the other. The 

 mouth is usually armed with jaws, and the opposite extremity of the intestinal canal always ter- 

 minates in an anal opening. The sexes are usually distinct, although a few, as the leeches and 

 earth-worms, are hermaphrodites. Some species appear to propagate by spontaneous division, 

 and many of them can reproduce parts lost by accidental injury. These animals include four 

 orders, the Errantia, Tubicola, Scolecina, and Suctoria. 



