GENERAL CHAnACTERS OF WOEMS. 37 



The bodies of worms are typically elongated, cylindrical, and 

 soft, adapted to live in damp media. We can always dis- 

 tinguish a dorsal or upper and a ventral or lower surface. 

 Some worms are flat, and are known as Platyhelminthes ; others 

 are round, and are called Nemafhelmintlies : these round worms 

 are never segmented. A third group are round or nearly so, 

 and are more or less segmented ; these are known as Ghietopoda 

 and Hirudiiiea. 



In the more highly developed worms the two anterior seg- 

 ments unite to form a head. The skin is very variable, and 

 covers a strong muscular layer or coat. The cuticle, or outer 

 layer of the skin, may become very thick, and then forms 

 a kind of exoskeleton. The layers under the skin, the true 

 cutis, contain circular and longitudinal muscle-fibres : these pro- 

 duce a thick layer, and enable the worms to move with great 

 power by their muscular contractions. The internal organisa- 

 tion of this class of invertebrates is also very variable. In such 

 worms as are parasitic and live on the juices of other animals, 

 the alimentary or digestive canal may be absent (Cestodes). 

 Nutrition then takes place much as in plants — namely, by the 

 process of endosmosis ; the nutritive fluids are absorbed through 

 the skin. When an alimentary canal is present we find that 

 the mouth is always ventral, and that there may {Nematoda) or 

 may not {Trematoda) be a posterior exit, the anus. The mouth 

 generally gives rise to a muscular pharynx, which opens into 

 a well-developed stomach, followed by an intestine of very 

 variable length and structure, a short rectum uniting the latter 

 with the anus. 



In the worms we find a well - developed nervous system. 

 In the most simple form seen in these invertebrates there is 

 only a single or paired mass of nerve-cells, the so-called ganglia, 

 over the gullet or oesophagus. These ganglia are called the 

 " brain," and from them proceed nerve-fibres, some going an- 

 teriorly to the sense organs, others posteriorly to the skin, &c. 

 In Chsetopods, arising from the ventral or sub -oesophageal 



