444 



LECTURE XIX. 



The body is divided into two principal parts, of which the anterior 

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

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

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

 the first, from which it is divided by a deep constriction ; 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 con- 

 sist 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 hair-follicles of 

 the human skin, {Demodex folliculorum, Jig. 162)*, 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 Anellids to the higher Articulata. In length, it ranges 

 from y^^th to y^th of an inch. Fig. 161 gives a magnified view of 

 the human hair-follicle (a), containing the bulb of the hair (b), the 

 appended sebaceous sac (c), and the duct {d) containing the parasitic 

 Arachnidan in question (e). That this parasite ranks with the 



162 6 



mm 



Demodex folliculorum in situ. Demodex folliculorum magnified. 



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

 ganised worms, is evident from the division of the body into thorax 

 and abdomen, from the structure of the head and mouth, which are 

 confluent with the thorax, and from the undivided abdomen. The 

 thoracic appendages {fig' 162, c, c), eight in number, as in the 

 Arachnids, are however of the simplest and most rudimcntal kind, 



CCCLXXXLp. 218. pl.x.i. 



