VIII 



AETHEOPODA 



231 



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

 in all essential features 

 has been confirmed by 

 Kautsch (Fig. 181). In 

 the case of the second 

 pair of legs the invagina- 

 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. 

 The third pair of spin- 

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



mass of the last pair of 



Fio. 180. — Sagittal sectiou through the hinder part of 



the abdomen of Agelena lahyrintMca to show the 



hinder part of the mid-gut developing in connection 



with the stercoral poolcet. (After Kautsch.) 



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



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



ventral longitudinal muscles. 



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 

 point. These grooves 

 now begin to be closed 

 in from the exterior, 

 and the last portions to 

 remain open are the 

 most posterior parts of 

 their inner limbs. Just 

 above the spots where 

 these grooves finally close, a pair of ectodermic invaginations mark 



Fig. 181. — Longitudinal section through the abdominal 

 appendages of an embryo of Agelena labyrinthica, 

 to show the origin of the lung book. (After 

 Kautsch.) 



a&ij a&2, a63, the first, second, and third abdominal appen- 

 dages respectively ; cogi, co^, coe3, the coelomic sacs belonging 

 to the first three abdominal segments ; l.l, lung lamellae ; hs, 

 lung sac. 



