METAZOA ARACHNOZOA. 367 



The single eyes are in the anterior part of the cephalo- 

 thorax, but in the scorpion there are three pairs on the 

 anterior edge and one pair situated farther back. 



The number of appendages attached to the cephalo- 

 thorax and abdomen is the same (Nos. 905,906). These 

 consist of a pair of mandibles, or chelae as they are called 

 in the scorpion ; a pair of maxillae or pedipalpi, provided 

 in the scorpion with nippers ; and four pairs of similar 

 walking-legs (in the preparation one leg is wanting). 

 The first pair of abdominal appendages are represented 

 in both animals by the operculum which conceals the 

 median genital opening. The second pair have become 

 peculiarly modified in the scorpion to form comb-like 

 organs (see No. 906) which are probably tactile in func- 

 tion (Shipley). The four following abdominal segments 

 of the scorpion have slit-like openings or spiracles which 

 lead into sacs containing the breathing organs or lung- 

 books. There are four pairs of these internal organs 

 (No. 906), each consisting of hundreds of leaves. The 

 embryo scorpion has external appendages on these seg- 

 ments but in the process of development they are re- 

 placed by the lung sacs. The latter organs therefore 

 correspond to the last four pairs of external branchiae of 

 Limulus. 



The terminal, postanal section of the scorpion is pro- 

 vided with two poison glands, and the poison is dis- 

 charged through the opening at the end of the sharp, 

 pointed spine which is in reality the poison fang. 



The spiders are the most specialized of the Arachnida. 

 This specialization is shown most strikingly in the adult 

 condition, while the embryo spider exhibits certain char- 

 acters possessed by the scorpions and probably by the 

 earliest ancestors of the group. The adult is without 

 even vestiges of abdominal legs, but the embryo has them 

 clearly marked (PI. 907, fig. i, Clubione coiled, and fig. 

 2, the same unrolled) as six pairs of bud-like projections. 

 The four pairs of cephalothoracic feet and the two pairs 



