3 1 8 MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



Ill 1871 E. Van Beneden first suggested the homology between the branchial limbs of Limulus 

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

 this view. In his second paper he derives the Arachnida from the Pojcilopodes (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 into lungs : the special details of this process of transformation are not wholly under, 

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

 the inner germlayer are changed to blood-corpuscles. The third and fourth pairs of abdominal legs form the germs of the spinnerets; 

 between the third pair 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. MUne-Edward thus refers 

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



Les membres abdominaux des Limules sont, comme on le salt, flargis en forme de lames, et ceux de la premii'^re 

 palre, tout en servant d'opercule potu- clore en dessous la fosse respiratoire, portent les orifices g^nitaus, tandis fiue 

 les membres des qnartre paires sulvantes donnent naissance a autant de branches multifoli^es. Chez les scorpions, 

 il n'y a rien (lui rappelle les appendices operculiformes dont je viens de parler, et les orifices g^nitanx sont situds un 

 pen plus en avant h la partie sternale de la r(Sgion thoraciriue ; mats 11 y a une grande ressemblance entre les cinq 

 paires de fausses pattes brauchiales des Limules et les quatre paires de poches pulmonaires des scorpions; 11 y aurait 

 meme presque identity si, chez des Limules, ces appendices, au lieu d'etre libres par leurs bords latc^raux aussi bien 

 qu'eu dessous, contractaieu t avec les parties voisines du test des adhc^rences, de fajon a ne laisser d'ouverture que 

 sous leur bord infdrienr, et si les feulUcts branchiaux de ces animaux, au lieu d'etre imperfor^s, se creusaient d'une 

 cavitfi accessible i I'air, ;\ peupres de la meme manifere que les fausses pattes branchiales des Tylos et des Porcelliens 

 se creusent de poches pulmonaires. Si la forme organique realis<5e par les Limulus, au lieu d'etre appropride ft, la 

 vie aquatique, s'adaptait :\ la respiration adrienne d'luie mani&re analogue a ce que nous savons exister chez certains 

 reprdsentauts terrestres du type dont ddrivent les Crustac6s isopodes a respiration aquatique, il n'y aurait done, 

 sous ce rapport, aucuno difference importante entre ces deux sortes d' animaux articuliSs (p. 56). 



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

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

 ages, he remarks: 



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

 able, they remain as the spinning mammilhe, two pairs of which are now present. 



Balfour's observations were made on Agelena Jabyrhithica. 



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

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

 was apparently 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 ai)pendages, we find 

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

 apertures leading into pits correspondinii to the stigmata of the pulmonary sacs of Scorpio. 



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

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

 abandoned by the author. We opposed this hypothesis in a critique of Lankesters 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 scor])ions 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 scorpion as the homologues of the gills of Limulus, 

 but explained the mode of transformation of gills into book-lungs diflerently from Laukester. 



He regards the 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 

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

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



*He undoubtedly meant to say four pairs of rudimentary appendages, as he figures them (Figs. 5 to 8fl, 86). 



