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 maxillse ; 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 Myriapods, and also in the Crustacea, ac- 

 cording to Rathke's researches. Thus Avliile the ceplialothora- 

 cic rings appear simultaneous]}- the abdominal segments appear 

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

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

 extremities of the protozoouites have become enltirged ; these 

 enlargements form the appendages. Tliese tubercles, or rudi- 

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

 racic rings (Fig. 625). 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 

 young spider is at first like a caterpillar, having "false," de- 

 ciduous, abdominal legs. Five abdominal rings are i:)resent 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 ventral 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 permanently. Meanwhile the two cephalic 



*Ancl in like manner tlie replialic loljes, containinc- the ocelli, are seen in the 

 author's figures foldecl back upon the base of the heart, so that the antennae are 

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

 the ej'es. The stnictnre and snccession of the rings of the insectean head are 

 most readily exrilained, and some cine is given to their number and succession 

 by comparison with the embryo of spiders. 



