3 1 8 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



In 1871 E. Van Benedeu first suggested the homology between the branchial limbs of Limulus 

 and the lungs of Arachnids. (C. E. Soc. Ent. Belgique, 1871.) He afterwards (1882) reaftirmed 

 this view. In his second paper he derives the Arachnida from the Poecilopodes (Limulus). 



• Salensky, according to the abstract in Jahresb. lib. Anat. u. Phys., as early as 1871, stated 

 his belief that, in the spider whose embryology he studied, the third and fourth pairs of provisional 

 abdominal appendages became spinnerets^ while the two anterior pairs developed into lungs. 



The following is a translation of what is said by the reporter: 



The first pair of abdominal legs are transformed int-o lungs ; the special details of this process of transformation are not wholly under- 

 stood by the author. The second pair of abdominal legs become flatter, broader, and modified into a vascular sinus, in which the cells of 

 tile inner geriu-Iayer are changed to blood-corpuscles. The third and fourth pairs of abdominal legs foim the germs of the spinnerets ; 

 between the third p'air arise two new hook-like projections becoming a third pair of spinnerets (p. 324). 



In his valuable Recherches sur I'Auatomie des Limules (1872) A. Milne-Edward thus refers 

 to the homology between the gdls of Limulus and the lungs of the scorpion: 



Lea meuibres abilomiuaux des Limules soiit, eomme on le salt, ^largis en forme de lames, et ceux de la premifere 

 paire, tout eu servant d'opercule i)our clore eu dessous la fosse resplratoire, portent les orifices g^nitaux, tandis que 

 los membres des quartre paires suivantes dounent naissance ii autant de branches multifoliiSes. Chez les scorpions, 

 il n'ya rien qui rapjaelle les appendices operculiformes dont je viens de parler, et les orifices g^nitaus sent situ^s uu 

 })eu plus en avant :\ la partie sternale de la region thoraciquo ; mais il y a une grande ressemblance eutre les cinq 

 paires de fausses pattes branchiales des Limules et les quatre paires de poches pulmonaires des scorpions ; il y aurait 

 uicuie presque identity si, chez des Limules, ces appendices, an lieu d'etre libres par lours bords lat^raux aussi bien 

 qu'eu dessous, contractaieut aveo les parties voisines du test des adh^rences, de fa?ou a no laisser d'onvertnre quo 

 sous leur bord inffirieur, et si les feuillets branchiaux do ces animaux, au lieu d'etre imperfords, so creusalent d'une 

 cavitc accessible a I'air, a peuprfes de la meme manifere que les fausses pattes branchiales des Tylos et des Porcelliens 

 sc creusent de poches pulmonaires. Si la forme orgauique r(5alisde par les Limulus, au lieu d'etre appropriiSe k la 

 vie aquatique, s'adaptait :\ la respiration a^rieuno d'une manitre analogue !\ ce que nous Savons exister chez certains 

 rei)resontants terrestres du type dont d^rivent les Crustaccs isopodes il respiration aquatique, il n'y aurait done, 

 sous ce rapport, aucune diff(Srence importante outre ces deux sories d'animaux articulds (p. 56). 



Balfour, in his "Notes on the development of the Araneiua " (1880), does not refer to the book- 

 lungs or to their mode of origin. In speaking of the foiu- pairs of rudimentary abdominal append- 

 ages, he remarks : 



The four rudimentary appendages* have disappeared, unless, which to me seems in the highest degree improb- 

 able, they remain as tlie spinning mammillie, two pairs of which are now present. 



Balfour's observations were made on Agelena labyrinthica. 



In his essay entitled "Limulus an Arachnid," Lankester, in 1881, following A. Milne-Edwards, 

 adopts the liomology of the book-lungs of the Arachnida with the branchial legs of Limulus. He 

 wiiw ;il>i»ar(nt]y ignorant of, or had overlooked, Salensky's opinion that the last two pairs of em- 

 bryonic abdominal appendages of the spiders become the spinnerets. Lankester remarks: 



When we examine the sternal area of the segments of Limulus which carry lamelligerous appendages, we find 

 that, although the integument is mostly soft and flexible, yet there are small sclerites present, and in fact aligmata or 

 apevtures leading into pits corresponding to the stigmata of thepnlmnixirtj sacs of Scorpio. 



These "muscular stigmata" are tlien described in detail, followed by a hypothesis of the mode 

 of origin of the book-lungs from braiichifeious limbs like those of Limulus, which was afterwards 

 abandoned by the author. We opposed this hypothesis in a critique of Lankester's paper (Amer. 

 Naturalist, April, 1882) and suggested a difficulty in the acceptance of this view when the gill- 

 plates of the Eurypterida are taken into account, these being arranged somewhat like the teeth of 

 a rake. Lankester also expresses the opinion that the Arachnida through the scorpions were 

 derived from the Merostomata, and that it is not possible to place the scorpions and the Merosto- 

 mata in separate classes (p. 82). 



MacLeod (1882) regarded the lungs of the scori)ioii a.s the homologues of the gills of Limulus, 

 but explained the mode of transfoimation of gills into l)ook-luugs difl'erently from Lankester. 



He regards tlie two structures as homologous and explains the transformation of' the Limulus 

 gills into lungs in the following way: The lengthening of the abdomen of Limulus had the result 

 that the abdominal gill-bearing limbs were no longer covered; hence a part of each gill-bearing 

 liiiili coalesced with the integument of the abdomen and then respiratory cavities came to exist. 

 Wlicn this kind of Limulus became adapted to a terrestrial life, the gill-leaves, no longer supported 



He uudoubtedly meant to say ioui pairs of rudimentary appendages, as he figures them (Figs. 5 to 8a, 86). 



