No. 2.] THE EMBRYOLOGY OF LIMULUS. 239 



Between the lungs of the scorpion and the gills of Limuliis 

 the resemblances are closest. In Limulus the gills are borne 

 on appendages VIII-XII, in the scorpion upon appendages 

 IX-XII and in no Arachnid do tracheae occur behind this point. ^ 

 Farther, appendage VIII in the scorpion — the pecten — shows 

 plainly its homologies with its homologue in Limulus, the teeth 

 of the comb being the gill-leaves. This is exactly what we 

 should expect upon our hypothesis, for the scorpions, where the 

 resemblances are closest, are admitted by all to be the most 

 primitive of the Arachnids and which naturally should possess 

 the most ancestral type of respiratory organs. The other view, 

 that the lungs are modified tracheae,^ leads into considerable 

 difficulties for we then find the oldest stock, — the Stamm- 

 form of the Arachnida — possessing the most highly differ- 

 entiated organs of breathing, while in the most aberrant groups 

 the tracheae have been retained in an unmodified condition. 

 Again the Arachnida as a class, according to the observations 

 of Plateau ('86) and Berteaux ('90), show an entire absence of 

 those visible respiratory movements of the body wall which are 

 so characteristic of Hexapods and Chilopods, a fact in full 

 accordance with the thesis here maintained but not easily ex- 

 plained upon the standpoint of a common origin of all Arthropod 

 tracheae. 



Farther, the conversion of the gills directly into tracheal 

 tubes is at present going on in the case of the Oniscid Crustacea 

 where we have tubes lined with a chitinous intima penetrating 

 to the interior of the organ and conveying air to the blood. 



1 Bernard claims ('93) to have found traces of stigmata in the Pseudoscorpions 

 behind this point, but apparently his discovery is not a new one for von Siebold 

 pointed out, over forty years ago ('53, p. 370) that Bernard's predecessors had 

 also mistaken the cutaneous insertion of muscles for stigmata. 



2 This view is held by Sinclair ('92) who seems to ignore the possibility of there 

 being two kinds of tracheae; and, influenced by his observations upon the peculiar 

 dorsal tracheae of Scutigera, states his opinion " that we have a series from the 

 simple tracheae found in Peripatus up to the complete lungs of spiders which is 

 incapable of explanation in the present state of our knowledge, except as repre- 

 senting the stages of development of tracheae into the pulmonary organ of spiders." 

 A few lines lower he seems to think that the derivation of the lungs of scorpions 

 from gills implies a difference between spiders and scorpions greater than has been 

 supposed. 



