AEACHNIDA. 



251 



The Arachnida, like insects, are organised to live in air ; but they 

 are distinguished at first sight by the general form of the body and 

 the number of their legs, and by some important modifications of 

 their internal structure. The head is always, in the Arachnida, con«- 

 founded with the thorax, and is deprived of antennae, or at least of 

 homologous parts exclusively employed in sensation. They have 

 four pairs of legs. Some of the species respire by pulmonary sacs 

 only, in others these are associated with ramified tracheae^ and the 

 smaller Arachnidans breathe, like insects, by tracheae exclusively. The 

 dorsal vessel and a circulating system exist in all : the heart presents 

 a more compact and muscular form in the pulmonary Arachnidans. 



The integument is chitinous, as in insects, but presents the same 

 variations in density, in different species, as in the winged Articulata. 

 In the scorpions it is as dense and inextensible as in the Coleoptera : 

 in the spiders and mites it is generally softer than in insects, espe- 

 cially that of the abdomen. 



The body is divided into two principal parts, of which the antei-ior 

 is called the " cephalothorax," because it answers to the two first 

 segments of insects in a confluent state : the second and larger 

 division is called the abdomen ; it is generally larger and wider than 

 the first, from which it is divided by a deep consti'iction ; but in 

 scorpions it forms, as in Crustaceans, a slender continuation of the 

 thorax, a kind of caudal appendage divided into many joints. The 

 organs of locomotion are all attached to the cephalothorax, and 

 consist of eight legs, presenting different grades of development in the 

 different forms of the class, but, in most, being very similar to those 

 of insects, and almost always terminated by two hooks. 



The microscopic parasite of the sebaceous sacs and liair-follicles of 

 the human skin, lately discovered by Dr. Simon of Ber- 

 lin, and described in Miiller's Archiv. fiir Physiologic 

 (184'2, p. 218. pi. xi.), represents the lowest organised 

 form of the class Arachnida, and, like the parasitic 

 Cymothoe and Bopyrus of the Crustaceous class, 

 makes a transition from the Anellides to the higher 

 Articulata. In length, it ranges from J,th to -p,^|„tli 

 of an inch. Fig. 106. gives a magnified view of the 

 human hair-follicle («), containing the bulb of the 

 hair (b), the appended sebaceous sac (c), and the 

 duct (rf) containing the parasitic Arachnidan in ques- 

 tion (e). That this parasite ranks with the Arach- 

 nida, and not with the red-blooded or any of the lower 

 organised worms, is evident from the division of the 

 body into thorax and abdomen, from the structure of 



106 



Demodcx fol- 

 liculonim in situ. 



