DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 57 



now become marked off from one another (Fig. 30 B). The dorsal part 

 of the former is not extensive, and shows no traces of segmentation. 



The cephalic region has undergone considerable modification, the 

 cephalic lobes being smaller than before (Figs. 30 and 31). The 

 chelicerae are no longer post-oral, but have attained a pre-oral position 

 (Fig. 31 B). Between them the upper lip or rostrum has formed. 



The half segments, which became united dorsally, now join on the 

 ventral surface, after the ventral nerve-strand, which arose by the 

 thickening of the inner edge of the two halves of the germ-band, has 

 become detached and has been drawn inward. As has already been 

 mentioned, it is the anterior abdominal segments which, by their 

 active growth, give rise to the large spherical abdomen of the 

 Araneid. We might compare them to the pre-abdomen of the 

 Scorpion. The posterior segments, which are comparable to the post- 

 abdomen of the latter, however, degenerate either totally or to a great 

 extent. The long abdomen of the embryo gives place to the compact 

 abdomen known to us in the adult. 



An important point hitherto left unnoticed is the fate of the 

 abdominal appendages. These, as rudiments, resemble the thoracic 

 limbs, as can be seen from Fig 3 1 A ; while, however, the latter grow 

 and become jointed, the former decrease in size and become button- 

 like. During the flexure-reversal, and even after it, they retain the 

 same shape and position (Figs. 30 and 31 A). Then, however, they 

 begin to change. At the base of the limbs of the second segment, 

 the ectoderm becomes invaginated to form the lung-sacs, [and respira- 

 tory organs are similarly developed on the third segment. The fate 

 of the first abdominal segment is still uncertain. This segment, so 

 long overlooked (cf. p. 51, Fig. 27, 1, and footnote), is now recognised 

 by Erauer and Purcell as the homologue of that small and almost 

 limbless first abdominal segment which Brauer discovered in the 

 Scorpiones, and of the disputed segment bearing the metastoma in 

 Limulus (Ki.shixofye and Packard, Vol. ii., p. 345, footnote). If 

 this comparison is correct, the genital segment in all the three group* 

 is proved to he the eighth post-oral segment (i.e. the second abdominal 

 segment in the Scorpiones and the Araneae).~\ The third segment 

 (Brauer) of the Scorpiones yields the combs, which are structures 

 peculiar to this group. The fact that the segment which, in the 

 Scorpiones, carries the combs, may develop lungs in the Araneae is 

 explained by reference to Limulus, in which it carries gills.* 



The appendages of the third segment (see above) are said to 



* [Brauer, App. to Lit. on Scorpiones, No. II.. Purcell, App. to Lit. on 

 Araneae, No. VII.] 



