II. ACERATA: ARACHNIDA 389 



Order II. Eurypterida. 



Extinct Silurian and Devonian forms with small cephalothorax and large 

 twelve-jointed abdomen; intermediate between xiphosures and scorpions. 

 Eurypterus; Ptcrygotus, some species seven feet long. 



Sill) Class II. Araclmida. 



Under this name are included a number of orders of greater or less 

 extent which can be arranged around the spiders, or Aranea, as a centre. 

 There is considerable modification of form, and the follomng account 

 applies only to the more typical groups. In these the cephalothorax and 

 abdomen are separated by a distinct hne, and since the abdominal ap- 

 pendages almost entirely disappear in the adult, the number of abdomJnal 

 somites can only be ascertained where their boundaries are e\-ident. 

 The number varies between six in the phalangids and thirteen in the 

 scorpions. 



The cephalothorax is, except in the Solpugidffi, a single piece which 

 bears six pairs of appendages; the four posterior pairs, each typically 

 seven jointed, are locomotor, so that eight 

 legs are as characteristic for an arachnid as 

 ten for a decapod or six for a hexapod. The 

 first pair of appendages, the chelicerce (fig. 

 419), are preoral, the second, or pedipalpi, 

 beside the mouth. The chelicerffi are short 

 and consist of two or three joints, the terminal 

 joint either folding back upon the other or, 

 pincer-like, meeting an opposable thumb. In 

 the spiders the last joint or claw is forced fig. 419.— Mouth parts of 



into the prey, introducing poison from a sac Epeira. i, chehcerfe; 2, pedi- 

 ... rr^i 1- 1 • palpi; />, palpus; /, basal plate. 



in the basal jomt. The pedjpalpi are elon- 

 gate, leg-like, their basal joints often forming a lip, the other joints form- 

 ing the palpus, which may end with a claw or a pincer. 



The question has often been discussed as to whether the chelicerae are the 

 homologues of the antennas of other arthropods. The embryological evidence 

 is in favor of their equivalence to the second antenna of the Crustacea, and to the 

 mandibles of insects. In the cephalothorax is a fibrous entosternite to which 

 most of the muscles are attached, a structure paralleled in Limulus. 



Since the Arachnida usually suck their food, the cesophagus is fre- 

 quently widened to a sucking stomach, behind which comes the true 

 stomach, with wbdch, as well as with the intestine, a numlser of so-called 

 li\'er tubes may arise (fig. 415, da, dt). These may be restricted to the 

 abdomen, as in the scorpions. The hinder part of the intestine is often 



