32 GENERAL CHAKACTEKS OF VERMES. 



known as Platyhelminthes ; others are round, and are called 

 Nemathelminthes : these round worms are never segmented. A. 

 third group are round or nearly so, and are segmented ; these 

 are known as Annelids. 



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. 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 or may not 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. The Flukes (Trema- 

 toda) and some others, however, we shall see have no anus. 



Here in the worms we shall 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 

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

 In Annelids, arising from the ventral or sub-cesophageal ganglion, 

 are two nerve-strands : these in each segment of the worm swell 

 out into two ganglia, which are united by connecting nervous 

 matter called commissures. This chain of ganglia and the nerves 

 attached form the ventral nei've-cord. 



