52 



ARACHNIDA. 



The next modifications which take place can best be seen in the 

 unrolled germ-band of an Agalena (Fig. 28 A). The most striking 

 of these is the appearance of segmentation in the cephalo-thoracic 

 limbs. From the basal joint of the pedipalp, a longitudinal furrow 

 cuts off an anterior portion, which forms the masticatory blade, 

 while the remaining much longer and jointed portion represents 

 the palp (Schimkewitsch). The chelicerae remain for the time little 

 modified, but they also soon become jointed. In front of them a 

 new structure has appeared, the stomodaeum (st). Between the 

 cephalic lobes, and towards their posterior edge, a depression appears ; 



Fig. 28. — Various stages of the embryo of Agalena labyrinthica (after Bai.four, from -Lang's 

 Text-book). A and B, ideally unrolled. Between 6 and a the fairly wide true first abdominal 

 segment must be inserted, a, abdominal appendages ; an, spinning mammillae ; kl, cephalic 

 lobe (in B, with the semicircular groove ; between the two halves of the cephalic lobe is 

 the mouth, surrounded by the upper and the lower lips) ; st, stomodaeum ; 1, chelicerae ; 

 2, pedipalps ; 3-6, limbs. 



this at first is a pit, later a sac opening outwardly (Fig. 33, m and vd), 

 and represents the stomodaeum. Kear the mouth two paired structures 

 appear (Schimkewitsch) ; two small prominences, which lie anteriorly 

 near the oral aperture, and no doubt correspond to Croneberg's 

 antennae (p. Ill), unite immediately in front of the mouth to form 

 the unpaired lip or rostrum. Two similar prominences are said to 

 lie posteriorly to the oral aperture, and, fusing together, to yield a 

 kind of lower lip. These two, the powerful upper lip and the 

 slighter lower lip, the first forming an anterior and the second 

 a posterior semicircle, enclose between them the oral aperture 



