60 CHARACTERS OF ARACHNIDANS. 



so called, and consist merely in the formation of new rings and 

 a corresponding increase in the number of legs. 



The centipedes (Scoloperi dra) belong to this class. Most of 

 them live on the ground under stones and delight in the dark 

 (Jig. 55). 



LESSON V. 



CLASS OF ARACH'NIDANS. Organization Habits Classifica- 

 tion. 



ARACH'NIDA PULMONARIA. Aranei'dce or Spinners Mygale 

 Mason Spider Ara' nea sedenta'rice Ara'nea Vaga- 

 buridte Tareritula Scorpions. 



ARACH'NIDA TRACHEA'RIA. Mowers Aca 'rides Mites Itch 

 Arach'nidan Ticks. 



CLASS OF ARACH'NIDA. 



1. The class of Arach'nidans (from the Greek, arachen, spider) 

 is composed of animals, which, in their general organization, 

 resemble spiders. Like crusta'ceans and insects, they are articu- 

 lated animals with white blood (which is sufficient to distinguish 

 them from anne'lidans) ; but they differ from crusta'ceans, in the 

 fact that their aereal respiratory organs communicate externally 

 by means of openings called stigmata or spiracles, and they 

 differ from insects in the number of their legs, which is eight, in 

 the absence of a head distinct from the thorax, and, in general, 

 by the existence of a circulatory apparatus composed of arteries, 

 veins, and a dorsal vessel which performs the functions of a 

 heart. 



2. Most of these animals are of small size,, and the body is 

 divided into but two portions ; namely, a first part, consisting of 

 the head and thorax confounded in one piece (fig. 56) ; and a 

 second, consisting of the abdomen. 



3. The anterior portion or cephalo-thorax never bears antennre 

 as in other articulated animals ; in this part we observe, in front 

 and below, the mouth, which is furnished with mandibles ; the 

 jaw, almost always bearing palpi, and a lower lip; and pos- 

 teriorly, the legs, which in the adult number four pairs. Arach'- 

 nidans never have wings, and their abdomen, which is gene- 



1 . What are the characters of arachnidans ? 



2. How is the body divided ? 



3. What parts are borne by the cephalo-thorax ? 



