58 Invcrtcbrata. 



the most curious forms in the animal kingdom. They 

 are elongated, soft-bodied creatures, which have their 

 organs arranged in a bilaterally symmetrical manner. 

 In many of them the body consists of successive seg- 

 ments, arranged in a chain, each intermediate segment 

 being like its preceding and succeeding neighbours. 

 There is a nervous system in most forms, consisting 

 of two or more nerve knots about the pharynx and a 

 cord prolonged backwards along the under side of 

 the body, beneath the digestive canal. A water- 

 vascular system exists in some form or other in all 

 worms, but it has never any connexion with the 

 function of locomotion. It usually consists of a sys- 

 tem of tubes, one pair in each of the successive segments 

 of the body, opening by one extremity on the surface 

 and by the other end communicating with the body 

 cavity. These sets of tubes are commonly known 

 among the higher worms by the name of segmental 

 organs. Blood-vessels often exist, and sometimes con- 

 tain coloured blood, but there is no true heart, and the 

 colour does not depend on the existence of minute 

 floating coloured corpuscles in a colourless fluid, as 

 is the case with the blood of vertebrates. The com- 

 mon earth-worm and leech may be taken as well- 

 marked examples of the sub-kingdom. 



Many worms are parasitic and live within the 

 bodies of higher animals ; among these the circulatory, 

 water-vascular, and digestive systems become rudi- 

 mental, the nervous system remains undeveloped, the 

 body cavity often vanishes, and the reproductive 

 organs alone are fully represented. This sub-kingdom 

 includes the following classes : 



