402 



ARACHNIDA 



group from which it sprang is allied to the vertebrates and thus may be the 

 ancestor of this important class^ as is maintained by Patten and others.* 



The subclass contains the single genus Limulus. A few years ago, 

 however, a new classification was proposed subdividing this genus into 

 three, which were grouped in two subfamilies: this classification has not 

 been generally adopted. 



Limulus 0. F. Miiller. With the characters mentioned: 5 species, 

 of which 4 inhabit the eastern coast of Asia and its islands. 



-3 3- 



Fig. 639 — Limulus 

 polyphemus (Ship- 

 ley). A, dorsal as- 

 Apect; B, ventral 

 aspect. 1, median 

 eyes ; 2, lateral eyes ; 

 ?>, cephalothorax ; 4, abdomen ; 

 5, telson ; 6, mandibles ; 7, 

 mouth ; 8 legs ; 9, chilaria ; 

 10, operculum ; 11, gills, 



L. polyphemus (L.) (Fig. 639). Length up to 50 cm.; color dark 

 brown: eastern coast of North America from Nova Scotia to Florida; 

 common. 



Subclass 2. ARACHNIDA. 



With rare exceptions air-breathing, terrestrial animals, without 

 antennae and with a body consisting of a cephalothorax and an abdomen. 



External Structure.— The cephalothorax bears six pairs of appen- 

 dages, the mandibles or chelicerae, the pedipalps, and the four pairs of 

 walking legs. The abdomen is without locomotoiy appendages. The boun- 

 dary between these two body divisions is usually distinct, but in the mites it 

 is obliterated. In the spiders as well as the mites segmentation has 

 mostly disappeared and the body is short and compact. In contrast to 

 these forms are the scorpions, in Avhich the body is long and vermiform, 

 with distinct segmentation. In the Solpugida the head is distinct from the 

 thorax and bears the first three pairs of appendages. The mandibles 



See "The Evolution of the Vertebrates and Their Kin," by W. Patten, 1912. 



