CHAPTER IV. 



ARTICULATA. 



HE animals composing the sub-kingdom Ar- 

 ticulata are characterised by having the body 

 enclosed in a tunic, or integument, consisting 

 of a series of rings, segments, or joints, "articulated" 

 together by a flexible membrane. The lowest of 

 the order, Vermes, may be divided into four classes 

 which are generally distinguishable by their external 

 appearance. Of these, the first two are for the most 

 part parasitic, living in the interior of other animals, 

 few of which are, in fact, exempt from the visits of 

 these unwelcome guests. Of these, the first, the Flat- 

 worms (Platyelmia) have the body flattened, and gene- 

 rally more or less ovate and leaf-like ; these present the 

 greatest divergence from the articulate type. In the 

 second, the Nematelmia, or Round-worms, the body is 

 usually elongated and cylindrical, and the division into 

 segments is often indicated by annulations of the skin. 

 The other two classes are composed principally of aquatic 

 animals. One of these, the Rotifer a, or WJieel Animalcules, we have 

 already seen, includes a number of minute creatures. The Annelida, 

 or true worms, have the body distinctly divided into segments, gene- 

 rally furnished with lateral appendages, with a well-developed nervous 

 system. The blood, in this last class, is also generally of a red colour. 

 We shall first notice the Entozoa, or internal parasites ; few animals 

 can present a better claim to our notice, from the circumstance that 

 many of them find their natural residence in our own bodies, and in 

 those of our domestic animals, where they often do us a great amount 

 of injury. They have a still stronger claim to the attention of the 

 scientific zoologist, from the number of points connected with their 



