No. 6.] STRC/CTURES OF THE ANTENNATA. 



277 



Few observations have been made on the embryonic develop- 

 ment of these structures. Ayers ('85), in his account of Occan- 

 tJiHS idvcHS, describes a median ingrowth 

 between successive segments, which, as 

 he states, atrophies late and "at the time 

 of the closure of the dorsal wall of the 

 body there is seen between the connecting 

 cords of two adjacent pairs of ganglia a 

 small triangular or cylindrical mass of 

 cells, concerning the fate of which I am 

 uncertain. I believe, however, they go to 

 form a part of the internal skeleton. The fig. 3.— Two successive ventral 

 chitinous rods in the thoracic region, to -g--tsof.y../.A«,^r.,seen 



o ' irom above, snowing median 



which the muscles of the legs and wings deepening of the intersegmen- 



. tal depression. 



are attached, probably arise from the rem- 

 nant of this median invagination, but in the abdominal region 

 they may disappear entirely without giving rise to such struc- 

 tures." Wheeler ('93) describes and figures 

 a similar series of invaginations in the 

 Xiphidiuni embryo extending through the 

 thoracic and abdominal regions, of which he 

 says the former " are converted into the 

 chitinous apodematous structures, which give 

 attachment to some of the leg muscles." 

 The latter disappear. This, of course, is 

 just what we should expect if the abdominal 

 ingrowth originally served the same purpose 

 as the thoracic, but is now no longer func- 

 tional. It is interesting to note, too, that in 

 the embryos observed there occur evanescent 

 traces of muscle-like cords running from the 

 median ingrowth to the body wall (see figure). 

 Thus it appears that both comparative and 

 dinai section of embryo embryological study indicate that the apo- 



of Oecanthus niveus. y o .1, 



(After Ayers.) m.i.,m<t. dcmcs are parts of an originally metameric 

 dian invaginations. gystcm of chitinous invaginations extending 



throughout the body and supporting the leg- and body-wall 



musculature. 



Fig. 4. — Median longitu- 



