314 PROCEEDINGS OF THE ACADEMY OF [May, 



appendage does not invaginate, but disappears by merging with the 

 surrounding ectoblast, the trachea arises after it has disappeared, 

 the tracheal vestibulum is not formed from this appendage, and there 

 is no operculum. (2) The pulmonary stigma arises immediately 

 behind its appendage, but the tracheal stigma develops more or less 

 behind the original position of the appendage of its segment. (3) 

 The pulmonary lamellse arise from a thickened solid cell mass, which is 

 not the case with the tracheal branches. (4) The tracheae appear 

 somewhat later in the ontogeny. 



Thus while the anatomical differences are not great the ontogenetic 

 are considerable. The most important difference is that the lung- 

 books develop from appendages. On the basis of this particular 

 difference we must conclude that there is no complete homodynamy 

 between the two sets of organs, but an incomplete resemblance. The 

 trachea exhibits a simpler method of formation, but this in itself need 

 speak no more for primitiveness than for degeneracy. The present 

 evidence does not clearly indicate which is the more primitive. 



5. LlMULUS AND THE ARACHNIDS. 



To one question, however, we may take a more decided stand — that, 

 namely, of the reference of aranead lung-books to the branchiae of 

 Limulus. Since Straus-Durckheim (1829) first insisted that Limulus 

 is much more closely related to Arachnids than to Crustaceans, this 

 view has grown steadily in support, especially in the hands of E. Van 

 Beneden (1871), A. Milne-Edwards (1873), Barrois (1878), Lankester 

 (1881), Patten (1890) and Kingsley (1893). And there can be little 

 question that Limulus and the Arachnids are related, from the corre- 

 spondence in segmentation, arrangement and number of appendages, 

 endosternal and entapophysial structures, central nervous and vascular 

 systems and other organs, all of which have been carefully compared 

 by Lankester and Kingsley. The principal difference between the 

 two has been considered the respiratory organs, and Lankester sought 

 to obviate this by reasoning that the lung-books of Arachnids arose by 

 the invagination of lamelligerous branchial appendages, and that the 

 tracheae are in their turn modifications of lung-books. This view 

 seemed to be confirmed by the accounts of the development of arach- 

 nidan lung-books as given for spiders by Kishinouye, Purcell and Sim- 

 mons, and for the scorpion by Brauer (1895). 6 



6 Brauer's study did not determine the first origin of the pulmonary lamellse , 

 tor there is a gap between the stages shown in his text figs. 156 and c. 



