NEMATODES. 181 



Hum is altogether absent. From a nerve ring around the 

 pharynx six nerves run forwards and six backwards. An 

 alimentary canal r , consisting of fore, mid, and hind gut, is 

 usually developed. There is no vascular nor respiratory 

 system, but there is usually a body cavity, and there are two 

 excretory tubes opening by an anterior ventral pore. The 

 sexes are usually separate and the reproductive organs simple. 

 The life history is often intricate. 



Form. 



The body is usually cylindrical in cross section and 

 tapering at each end. The male is usually smaller than 

 the female, and his tail, concerned in copulation, bears 

 sensory papillae, and usually some spines and a " bursa." 



Body Wall. 



(a.) Most externally there is a chitinoid, often wrinkled, 

 cuticle, thick in the larger forms, and perhaps of ser- 

 vice in enabling the animals to resist drought and digestive 

 juices. With its presence may be associated the almost 

 entire absence of cutaneous glands, and the entire absence 

 of cilia, (b.) Beneath this is the sub-cuticula or hypodermis, 

 usually thickened in four longitudinal lines median dorsal, 

 ventral, and lateral. (c.) Beneath the hypodermis is a 

 layer of longitudinal muscles, which sometimes lie in groups 

 defined by the above mentioned lines. Many of the 

 Nematodes are very agile. 



Nervous System. 



Around the pharynx there is a nerve ring from which 

 six nerves run forwards and six backwards. One of the 

 latter runs along the median dorsal line a unique 

 position in an Invertebrate. Here and there on the 

 ring and on the nerves there are ganglionic cells, but any 

 aggregation of these into ganglia is rare. Some of the free 

 living forms have eye spots ; and probably all Nematodes 

 have sensory papillae on various parts of the body. 



Alimentary System. 



The mouth is terminal or almost terminal ; the anus 

 is ventral and posterior, and occasionally terminal. As 



