322 MEMOIES OP THE NATIONAL ACADEMY OF SCIENCES. 



me that the book-leaves of Arachiiida are really the homologies of the thin gill-sacs of Limulus, and 

 that the two classes have probably originated, if not from a common ancestor, yet from those stand- 

 ing very near each other. 



In closing, we would make a suggestion in the way of an explanation of the points of resem- 

 blance of the Podostomata (Merostomata and Trilobites) to the Phyllopod Crustacea, especially 

 Ai)us. Even so strong an opponent of the idea that there is any close affinity between Apus and 

 Limnlns as Laukester, allows that there ai-e some points of resemblance between Apus and Limu- 

 lus, "the most important being the jaw-processes at the base of all the ambulatory limbs, and the 

 presence of a pure archicerebrum in both genera." 



We have shown that the brain of Limulus, though roughly speaking au archicerebrum, is 

 very different fi'om that of any Phyllopod. Still the points of resemblance between these 

 primitive Phyllopods and the Merostomata are due, perhaps, to the fact that the ancestors of both 

 Crustacea and Podostomata were probably derived from somewhat similar Annelid-like ancestors, 

 though by qirite indei)endent lines of descent. We would therefore agree with Lankester's con- 

 clusion in his essay on Apus, that the points in which Apus and Limulus agree " are ^irobably 

 points in which they both approach the common ancestor of the Arachnida and Crustacea." 



But we dilfer from this able author in supposing that the Merostomata and Trilobites, or at 

 all events the Merostomata, should be merged with the Arachnida, as they have characters, both 

 external and internal, which forbid their being I'egarded as Arachnids. 



We believe, with A. Milne Edwards, that they should form a separate class. There were, then, 

 four lines of development in the Arthropoda (throwing out for the present the Linguatulina and 

 Tardigfada), viz: The Podostomatous line, the first to be struck ofi'from the Annelidan stock (the 

 trilobites being the first forms to appear), second the Arachnidan line; third, the Crustacean line, 

 nearly coeval with the first or Podostomatous; and the fourth, the line culminating in Myriopods, 

 Scolopendrella, and insects; and it is safe to suppose that the terrestrial tracheate groups of Arach- 

 nida, Myriopoda, and insects were later products than the marine, aquatic branchiate classes, 

 i. e., the Podostomata and the Crustacea. 



LITEKATIRK HEARING ON THE SUPPOSED HOMOLOGY OP THE BKANCHIAL APPENDAGES OF LIMULUS AND THE BCOK- 



LUNGS OF ARACHNIDA. 



Balfour, F. M. Notes on the developiimnt of the Araueiua. Quart. Jour. Micr. Science, xx, 1880. 



Hernari), H. M. An endeavor to show that the trache;B of the Arthropoda arose from setiparous sacs. Spen- 

 gel's Zoiil. .lahrhuch, pp. 511-524, 1S92. 



Bektkau, Ph. IJeber die Respiratiousorgane der Araneen. Arch, flir Naturgeschichte, .38. Bd., 1872. 



KiNG.si.EV, .1. S. Notes on the (embryology of Limulus. Quart. Journ. Micr. Science, xxv, 1885. 



KisHiNOUYE, K. Ou the development of Araneina. .Jour. College of Science Imp. University, Japan, iv, 1S90. 



. On the <levelopment of lAmtdiis loiigispinn. Ibid. v. 1891. 



Lankester, E. R. Limulus an Ar.achnid. Quart. Jour. Micr. Science xxi, 1881. 



. New hypothesis as to the relationship of the lung-book of Scorpio to the gill-book of Liiniilus. Quart. 



Jour. Micr. Science, xxv. p. 339-342, 1888. 



Laurie, M. On the development of the lung-books in Scorpio fulvipes. Zool. Anzeiger. xv. .lahrg., No. 386, 14 

 Miirz, 1892. 



LocY, \V. A. Observations on the development of Ageleue u.-evia. Bull. Mns. Comp. Zool. xii, 1886. 



MacLEOD, J. Recherches sur la structure et la signification de I'appareil respiratoire des Arachnides. Archives 

 de Biologie, Gand, Tome v, 1884. 



Milne-Edwards, A. Recherches sur I'anatomie des Limulus. Annales des Sciences Naturelles, 5mo S^r. Zoolo- 

 gie, xvii, 1873, Paris. 



Metschnikoff, E. Embryologie des Scorpions. Zeit. fiir Wissensch. Zoologie xx, 1870. 



Patten, W. On the origin of Vertebrates from Arachnids. Quart. Journ. Micr. Science xxxi, New Ser. 1890. 



Saxensky, W. Embryology of the Araneidse. Mem. (Sapisky) Kieff Soc. of Naturalists, ii, Pt. 1, pp. 1-72, PI. 

 i-iii. 1871, Kieff, 1871. (Russian). Abstract in Jahresb. iiber Anat. u. Physiol. (Hoffmann u. Schwalbe), Bd. ii, 1873, 

 pp. 323-325, 1875. 



ScHiMKEWiTSCii, W. ]5tude suT le d<5veloppement des Araign^es. Archives de Biologie, vii, 1885. Bruxelles. 



Van Beneden, E. De la place que les Limules doivent occuper dans la classification des Arthropodes. C. R. 

 Soc. Ent. de Belgique. Oct., 1871. (Ann. .and Mag. Nat. Hist. 1872.) 



. Sur la structure et la signification de I'appareil respiratoire des Arachnides. Bull. Scient. Depart. 



(In Nord, 5 'ann., pp. 299-301, 1882. 



Walcott, C. D. The Trilobite: New and old evidence relating to its organization. Bull. Mus. Corap. Zool. viii, 

 1881. (See also Walcott's article issued in advance of the 28th Report of New York State Museum of Nat. Hist. 1876, 

 also the 31st Report of the same museum.) 



