252 ANIMAL PHYSIOLOGY. [LESSON 76. 



1136. These animals present a more concentrated form of the 

 nervous system and of the heart than the animals previously exam- 

 ined; the larger species likewise offer a higher condition of the 

 respiratory system, which is less diffused than in Insects, and in some 

 consists only of air sacs, or lungs. 



1137. But the most essential mark of the superiority of the 

 Arachnida is the course of their development. The spider under- 

 goes no metamorphoses comparable with those of insects. It is at 

 no period of its development an apodal (legless) worm. 



1138. The head is always, in the Arachnida, soldered to the 

 chest, which thus forms the cephalo-thorax before alluded to ; it is 

 also deprived of antennae. 



1139. They all ha vefour pairs of legs, which of itself forms an 

 important characteristic ; thus, an Insect is divisible into three chief 

 portions of its body, head, chest, and abdomen, to which neither 

 more nor less than three pairs of legs are superadded. 



1140. The Crustacea, like the Arachnida, are divided into only 

 two portions, head-chest and abdomen, the legs being never less than 

 .five pairs, and frequently a greater number. 



1141. The majority, but not all of these animals, are, like the 

 Insects, air-breathing ; to this rule there are, however, some excep- 

 tions amongst the Mites, and certain Spiders. 



1142. The Mites, as their common name imports, are exceedingly 

 minute ; many of them, too, are parasitic. 



1143. Man is the victim of two species of them, one the Demodex 

 folliculorum (Figs. 245-'6-'7), being by far the most minute that 

 has yet been discovered, and the other is the Acarus scabisei, or 

 the itch animal. 



1144. That most loathsome of all diseases which afflicts humanity, 

 the itch, and the mange in dogs, are both caused by the presence of 

 a microscopical mite ; the species, however, in man and the dog, differ. 



1145. The animal which afflicts the human family is known as the 

 Acarus scabicei, or by its more modern name of Sar copies galei. 

 An illustration of this animal is given (Fig. 248). 



1146. This disease never occurs in any but very dirty people ; 

 through unpardonable neglect of washing the skin, the accumulation 

 of matter constantly being thrown off from its surface, produces 

 disease, and this disgusting parasite, when all other means are 

 neglected, comes, as a physician, to effect a cure ; this process is, 

 however, generally a lengthy one, until the aid of a physician of 

 another kind be invoked. 



