Development of the Lungs of Spiders. 211 



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 

 trachea? 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" (1881 a ) he 

 announced 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 " (188 l b ) he 

 formulates 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 unim- 

 portant points. To obtain the condition occurring in Scorpio 

 he assumes that the cup-like depressions behind the append- 

 ages, 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 invagi- 

 nation, 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 

 been converted into a blood-space. The atrophy of the 

 muscles which move the gills in Limulus and similar forms 

 was considered very essential to this theory. The difficulties 

 involved in the changes of blood- and air-spaces were so con- 

 siderable as to prevent the acceptance of this hypothesis. 



Later Lankester (1885) put forth a new theory. Because 

 of discoveries concerning the muscles (veno pericardiac) of 

 Scorpio, as well as on account of the insuperable difficulties of 

 his previous view, he gave up his old and advanced a new 

 hypothesis. In the latter the common ancestor is assumed to 

 have had six pairs of mesosomatic appendages, of which five 



