VIII 



AETHKOPODA 



231 



the development of the lung is due to Parcel! (1909), whose account 



in all essential features 



has been confirmed by 



Kautsch (Fig. 181). In 



the case of the second 



pair of legs the invagiua- 



tion gives rise to the 



lateral trachea. The 



median trachea arises as 



a modification of the 



entopophysis or ecto- 



dermal tendon of the 



longitudinal abdominal 



muscle. The first two 



persisting abdominal 



segments are very broad, 



and hence the hinder 



segments are forced back 



to near the posterior 



end of the abdomen. 



The two hinder pairs of 



abdominal appendages 



become the spinnerets 



and in each a solid ecto- 



dermic invagination is 



formed at the apex of 



the limb, and gives rise 



to the spinning glands. 



muse 



dm. 



'USC 



FIG. ISO. Sagittal section through the hinder part of 

 the abdomen of Agdena, lahi/rinthica to show the 

 hinder part of the mid-gut developing in connection 

 with the stercoral pocket. (After Kautsch.) 



d.Miixc, dorsal longitudinal muscles ; m.g, rudiment of hinder 

 part of mid-gut; sp, spinneret; sp.gl, spinning gland; 

 ventral longitudinal muscles. 



The third pair of spin- 

 nerets are formed by the division of an inner lobe from the main 



mass of the last pair of 

 abdominal appendages. 

 They appear after birth 

 (Fig. 182, C). 



The cephalic lobes 

 have now fused in the 

 middle line, and their 

 semicircular grooves 

 have also fused at one 



coe 



eve 2 



FIG. 181. Longitudinal section through the abdominal 

 appendages of an embryo of Ayelena labyrinth ini, 

 to show the origin of the lung book. (After 

 Kautsch.) 



ab l , ab-, u&3, the first, second, and third abdominal appen- 



grooves 



point. These 

 now begin to be closed 

 in from the exterior, 

 and the last portions to 

 remain open are the 



dages respectively ; coel, coe^, coeS, the coelouiic sacs belonging lllOSt posterior pai'tS of 

 to the tirst three abdominal segments; /./, lung lamellae; l.s, fligiv inner lilllbs JtlSt 

 lung sac. 



above the spots where 

 these grooves finally close, a pair of ectodermic invaginations mark 



