MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 319 



by the water, umst have simultaneously become soldered by their edges to the walls of the gill- 

 chanibcr. 



For our earliest information regarding tke mode of development of tlie lungs of spiders we are 

 indebted to Schimkewitsch (1885). He states: 



Balfour has showu * that the trache.-p and lungs of Araueina are formed by the invagination of the ectoderm. I 

 .im nnable to confirm this observation. In the embryo of Li/cosa saccata, before hatching, the trachejc are represented 

 by a jirincipal trunk from (vhich arise four secondary brandies. Their wall is formed of the epithelial ectodermic 

 layer lined by a homogeneous internal coat and covered by an external coat containing nuclei of mesodermal origin 

 (V, chap. III). In this same stage the structure of the lungs is very interesting (PI. XXII, Fig. l,pm). A. Milne- 

 Eihvards and J. MacLeod have endeavored to prove the homology of the lungs of Araohuida with the gills of Limuli. 

 The lungs of the embryo of Lycosa saccata consist of true trache:^ arranged in bundles; the principal trunk (PI. XXII, 

 Fig. 1), which opens outward by the stigmatic orifice (st), divides into five secondary branches. The epithelial 

 layer of these last is represented by flattened cells. The secondary branches, flattened from above downward, enter 

 the cavity surrounding the lungs, and in the spaces between these are to be seen the blood corpuscles. Thus the em- 

 bryonic lung consists of a trachea arranged enfaisceau; the only dift'erence to be noted is the absence of the external 

 tunic, which I have not observed, either on the lung of the embryo or on that of the adult (p. 561). 



In the following year, in his "Les Arachnides et leurs Afflnitcs" (188()), he reiterates this view. 



In the same year, Locy, in his "Observations on the development of Ar/eleiia nwvia," gives a still 

 more complete account of the mode of origin of the lungs of spiders, with excellent figures. He 

 states that the lungs arise as a pair of extensive invaginations at about the same time as the proc- 

 todiBum. " In sagittal sections of early stages the lungs appear as oblong plates of cells, the large 

 oval nuclei of which are arranged in parallel rows." The flattened sacs are hollow and divided at 

 intervals by 2cell columns, the interspaces sometimes containing blood corpuscles. The details are 

 mostly histological and the author does not enter into any morphological comparison with the book- 

 lungs of Limulus, but nothing is said contrary to the view that the luugs are a bundle of tracheiB. 



In his paper "On the origin of vertebrates from Arachnids " (1890), Patten refers to the chitin- 

 lined tubes of the scorpion, " which serve for the support of muscles." These, as we understand 

 them, are the "muscular stigmata" of Lankester and occur in insects as well as in Arachnida and 

 Podostomata, as we have observed them in a specimen of Asaphus as well as in Liuuilus. As 

 Patten remarks : "The chitiuized tubes are comparable with the three or four pairs of tracheal 

 invaginations which in insects give rise to the tentorium." He then adds : 



I regard the lung-books of Scorpio and the chitiu-lined tubes descrilted above as belonging to the same category, 

 for after careful study I have found nothing to indicate that they arise as modifications of rudimentary abdominal 

 appendages (p. 355). 



In 1885 Lankester discards his first hypothesis, which he regards as "overstrained," and 

 replaces it by a second "perfectly simple" one. 



In his notes on the embryology of Limulus (1885) Kingsley describes the early stages of the 

 abdominal limbs and discusses at some length their homology with the "pulmonary books" of 

 Arachnids, and he admits its general validity, though not ready "to follow all of Prof. Lankester's 

 intermediate steps, nor those of MacLeod." His hypothesis and figures are ingenious and more 

 simple than those of Lankester. As he remarks : 



As I have mentioned above, the process of formation of the gill-leaves is largely by a process of outorowth 

 but there is also a slight ingrowth, especially noticeable at the distal portion of the appendage. This, however, dis- 

 appears vrith growth, but is very noticeable in all my sections. To transform the gill of Limulus into the lung of 

 Scorpio it is only necessary that, together with the sinking of the whole organ, as described above, the inpushings 

 of the integument to form the lamellae should be exaggerated and the outgrowth correspondingly decreased. On 

 PI. XXXVII, Figs. 18 to 20, I have diagrammatically illustrated the steps in the process, the gill-leaves beiug few in 

 number to secure clearness. In 18 wo have the typical condition found in Limulus, one appendage being shown half 

 in section and half in perspective. In 19 we have an intermediate condition, when, as suggested above, the animal 

 was leaving the water and seeking a terrestrial life. Here the gill-bearing appendage (ya. ) is partially sunk in the 

 surrounding tissues to secure protection. The same causes would also tend to produce a similar change in the gill 

 leaves ((/l.) and they would also tend to be formed rather as ingrowths than <as protruding processes. This change 

 in structure would be the more readily eftected on account of a change of the medium of respiration. A gill needs 

 either to project into the water or to have some means of instantly clianging the fluid which bathes it. An organ 

 for aerial respiration, on the other' hand, is not so restricted in its position, since the air is more fluid and more 

 elastic and hence more readily changed. Another advantage to the animal resulting from the change is that the 

 oxygen is thus brought nearer to the tissues requiring it. 



* I can find no reference in Balfour's memoir to this subject. 



