Development of the Lungs of Spiders. 215 



stage the pectinfe and lung-book appendages are about equal 

 in size and structure. In stage L tlie pectines have become 

 folded in a direction transverse to their axes. The other 

 appendages are pushed in, forming little cavities (directed 

 forward) on the posterior sides of the appendages. In the 

 stage M the pectines are separated from the body-wall at their 

 distal ends. The lung-book inpushings are deeper and the 

 cavities are divided up by lamellse. In the last stage described 

 (just before hatching) the pectines and lung-books have much 

 the same structure as in the adult. 



Morin (1888)* states that the lungs of the dipncumonous 

 spiders arise in form of infoldings at the bases of the two 

 appendages of the second abdominal segment. At the ante- 

 rior end of the sac on the dorsal side is an infolding, which 

 is the beginning of the lung-leaves. The space between two 

 leaves connects directly with the body-cavity. Two adjoining 

 leaves unite by the fusing of cells, as described by Locy. He 

 agrees with Locy as to the later stages. Morphologically the 

 lungs of Arachnida show great resemblance to the gills of 

 Limulus and similar forms. He emphasizes the position of 

 the infoldings on the posterior side of the appendage in both 

 cases. The lungs of spiders are merely sunken gill-books of 

 Limulus. As the appendage sinks the stigma is left as an 

 opening between the posterior wall of the appendage and the 

 body-wall. This author agrees with those who believe that 

 tracheaa are modified lungs. 



It must not be forgotten that Elias MetschnikofF (1870) 

 described some features of the lungs of the Scorpions ; but it 

 is not easy to understand either his text or his figures beyond 

 the fact that he states that the lungs develop behind the 

 abdominal limbs. 



As will be seen from the foregoing summary, the develop- 

 ment of the respiratory organs of the Arachnids has not been 

 lollowed throughout, and the gaps in our knowledge are at 

 just the most critical points. To supply these deficiencies the 

 investigations described below were undertaken. 



1'he work was done in the Biological Laboratory of Tufts 

 College. The eggs used were those of Agelena ncevia and 

 Theridium teindariorum. The eggs were killed in water 

 heated to 80° C. and hardened in alcohol, beginning with 

 50 per cent. The staining was usually in toto with alum 

 cochineal. The sections described, unless otherwise specified, 

 are sagittal. 



* As summarized by Korseliclt and Heider (1892), pp. G04-607. I 

 cannot refer to the oris^inal text. 



