0. L. Simmons — Development of Lungs of Spiders. 119 



Aet. XV. — Development of the Lungs of Spiders / by 

 Orville L. Simmons. (With Plate VIII.) 



Although several persons* had suggested the close affinity 

 of the Arachnids and Limulus, it was not until the appearance 

 of Lankester's paper, " Limulus an Arachnid " ('81 b ) that the 

 view of such a relationship came into prominence. Since that 

 date it has attracted more and more numerous advocates until 

 now the majority of the special students of Arachnids and 

 Xiphosures recognize the close relationships of the two groups. 

 One of the special homologies insisted upon by Lankester was 

 that existing between the lungs of the Arachnids and the gills 

 of Limulus. But to explain the differences between these 

 organs — the one being an internal air-breathing structure, the 

 other an external apparatus for aquatic respiration — several 

 hypotheses have been advanced, all based upon conditions 

 existing in the adult. 



At first Lankester evidently shared the common view that 

 tracheae were homologous structures throughout the Arthro- 

 poda, and so he sought for traces of them in Limulus. In 

 his article " On stigmata in the King Crab " ('81 a ) he an- 

 nounced that he had found traces of stigmata. The position 

 of insertion of each thoracico-abdominal muscle is marked by a 

 deep funnel-like depression of the integument, which from the 

 external surface appears as a stigma. 



Later, in his paper " Limulus an Arachnid " ('81 b ) he formu- 

 lates an hypothesis to show how the gills of Limulus and the 

 lungs of Scorpio (taken because more primitive than Spiders) 

 could have been derived from a common ancestor which he 

 describes as being an aquatic form, breathing by book-like 

 gills. To derive Limulus from such a form would involve 

 only a few changes in dimensions and other unimportant 

 points. To obtain the condition occurring in Scorpio, he 

 assumes that the cup-like depressions behind the appendages, 

 as seen in Thelyphonus, became deeper and larger, finally 

 engulfing the whole appendage. The walls then gradually 

 extended over the cavity, leaving only a slit for communica- 

 tion with the exterior. As change of habits went on this slit 

 was closed up and another slit, still within the area formed by 

 the closure of the primitive opening of the cave of invagina- 

 tion, was formed. Air would enter by this slit, where in 

 Limulus and the early Scorpion ancestors there was blood 

 space. Thus a blood space has been changed to an air space. 

 In the same way an air space (that of the investing sac) has 



* Strauss-Durekheim (teste Lankester), van Beneden ('70). 



