636 ARACHNIDA. 



or primordial segments) takes place» Six of these zones or 

 segments arise between the cephalic and anal poles ; these 

 zones represent the ventral arcs. The two anterior rings bear 

 the mouth-parts, the mandibles and maxillae ; while the others 

 form rings corresponding to the four pair of feet. These pro- 

 tozoonites are very transitory, only existing for a short period ; 

 they gradually retreat towards the ventral side, enlarge and 

 nearly touch each other. 



The embryo (Fig. 624) now grows much longer, and new em- 

 bryonal segments are formed in the abdomen just as they grow 

 out in the worms, and IMyriapods, and also in the Crustacea, ac- 

 cording to Rathke's researches. Thus while the cephalothora- 

 cic rings appear simultaneously the abdominal segments appear 

 one after the other. The first one appears between the last tho- 

 racic ring and the anal " hood," or pole. Meanwhile the lateral 

 extremities of the protozoonites have become enlarged ; these 

 enlargements form the appendages. These tubercles, or rudi- 

 mentary limbs, appear on the abdominal as well as on the tho- 

 racic rings (Fig. G25). This fact is one of great interest, as 

 showing a resemblance to the Crustacean with its abdominal 

 legs, and more especially to the abdominal footed Myriapods, 

 and the larvae of many true six-footed insects. Thus the 

 3'oung spider is at first like a caterpillar, having "false," de- 

 ciduous, abdominal legs. Five abdominal rings are present in 

 Pholcus. 



Next follows the development of the "post-abdomen," or 

 tail, which being differentiated from the anal pole or "hood," 

 becomes detached from the yolk mass, and is folded back upon 

 the embryo, just as the abdomen of a crab is folded in an op- 

 posite way to the ^•entral side of the body.* 



This "post-abdomen," after dividing into three segments, 

 disappears completely during the growth of the embryo. This 

 is the more interesting, as the "post-abdomen" of the scor- 

 pion is retained permanentl3\ Meanwhile the two cephalic 



*And in like manner the cephalic lobes, containincr the ocelli, are seen in the 

 author's flarnres foUled back ujjon the base of the hear!, so that the antennaj are 

 never developed, and the mandibles of the spider take their place, in advance of 

 the eyes. The structure and succession of the rings of the insectean head are 

 most readily explained, and some clue is given to their number and succession 

 by comparison with the embryo of spiders. 



