632 THE MICROSCOPE. 



aerial respiration, their possession of four pairs of legs 

 attached to an anterior division of the body, and the total 

 absence of antennae. The body is also covered with a 

 softish skin, which sometimes attains a horny consistency, 

 but nothing more. In the higher forms, the body is 

 divided into two parts, the anterior of which, as in Crus- 

 tacea, consists of a thoracic segment, amalgamated with 

 the head, and forming together a whole called the cephalo- 

 thorax. In the highest classes the division of the thorax 

 into separate segments becomes apparent; the anterior 

 segment is, however, amalgamated with the head. The 

 structure of the abdomen varies greatly. In some cases 

 it forms a soft round mass, without any traces of seg- 

 mentation ' } whilst in others, as scorpions, it is continued 

 into a long flexible jointed tail. 



Acarea, an order of animals not strictly belonging to 

 insects, but rather to Arachnida, Spiders, Scorpions, &c. 

 The general description of the class is that the head 

 is united with the thorax, forming a cephalo-thorax ; no 

 antenna?, simple eyes, body presenting transverse striaB or 

 furrows between the second and third pair of legs, which 

 are eight in number, terminated by an acetabulum and 

 claws. These animals are commonly called mites, and 

 the best known species is that found in cheese, tliQAcarus 

 domesticus. Most of the species are oviparous and vivi- 

 parous, their eggs are very numerous. The spider enve- 

 lopes its eggs in a beautiful silken cocoon. Scorpions 

 produce their young alive, and it is deserving of notice 

 that in this family the embryo is developed in the ovum 

 while it still remains in the ovary. The existence of a 

 micropyle has not yet been made out in the ova of Arach- 

 nida. For the sake of convenience we have included 

 Parasites in this part of our work, and in Plate VI. Nos. 

 144 to 147 are representations of their eggs, from some of 

 which the larvas are just emerging. 



The Malophagus, or Sheep-tick, fig. 288, is apterous, 

 and seems to be a connecting link between acarina and 

 insects proper. The Sarcoptes scabiei produces the itch in 

 the human being : it is also found to be the cause of mange 

 in the dog. In one pustule on a dog suffering from this 

 disease, as many as thirty parasites have been found. 



