398 



latter. The cells of the thick epithelium, which contains the furrows, 

 shorten considerably and rearrange themselves in the form of a much 

 thinner and folded epithelium [cp 1 , Fig. 2) , so that the furrows 

 come to lie between the two walls of a fold directed inwards, while 

 the tissue between two furrows is that of a fold directed outwards. Of 

 the inwardly directed folds the oldest is the most medial and distal 

 one [ep 1 , i^ig. 2) ; it is already present in the stage of Fig. 1 , and is 

 represented in the adult by the most ventral of the hollow , air-con- 

 taining lung-leaves. The next lateral fold is the second oldest, and 

 so on. All grow inwards and subsequently nearly fill out the cavity 

 of the appendage. 



Hand in hand with the formation of folds, the pulmonary sack 

 [p.s^ Fig. 1) proliferates along the inner surface of the adjacent epi- 

 thelium in the direction of the arrow in Fig. 1, thus providing material 

 for the new folds. Finally, the whole appendage gradually sinks to the 

 level of the body-surface, whereby the region of the two oldest furrows, 

 yi,y2, becomes included in the enlarged pulmonary sack. 



Entapophyses and Tracheae. — The region [Eii Fig. 1 and 2) 

 of the epithelium adjoining the medial basal part of the posterior side 

 in all the abdominal appendages, forms the point of attachment of a 

 mesodermal, intermuscular tendon (endosternite), onto which are in- 

 serted the ends of the longitudinal ventral muscles and the ventral ends 

 of the dorsoventral muscles. The ectoderm at each point of attachment 

 becomes drawn out in the form of a hollow process, lined with chitin, 

 and projecting into the abdomen. These are the entapophyses or 

 ectodermal muscular tendons, and are serially homologous with one 

 another. 



The entapophyses connected with the first (pulmonary) pair of 

 appendages serve in the adult as the points of attachment of the most 

 anterior of the three well-known pairs of abdominal endosternites, 

 and may be quite separate from the pulmonary chamber {Lycosa); 

 or they may be connected with the latter and with one another by 

 means of the canal in the transverse fold joining the pulmonary 

 spiracles. 



The entapophyses connected with the second (tracheal) pair of 

 appendage become each drawn out into a long tube, to or near the 

 blind, inner end of which the middle pair of endosternites is attached 

 in the adult. These long tubes are represented by the two large trunks 

 which form the tracheae in the A t ti da e , and by the medial pair of the 

 four trunks which compose the tracheœ in most other Spiders (Agele- 

 nidae, Drassidae, Epeiridae, Lycosidae etc.) The homologon 

 of the lung is represented in the latter groups by the lateral pair of 



