ARACHNIDA

arachnida, the zoological name given in 1815 by lamarck (gr. [greek: harachnae], a spider) to a class which he instituted for the reception of the spiders, scorpions and mites, previously classified by linnaeus in the order aptera of his great group insecta. lamarck at the same time founded the class crustacea for the lobsters, crabs and water-fleas, also until then included in the order aptera of linnaeus. lamarck included the thysanura and the myriapoda in his class arachnida. the insecta of linnaeus was a group exactly equivalent to the arthropoda founded a hundred years later by siebold and stannius. it was thus reduced by lamarck in area, and made to comprise only the six-legged, wing-bearing "insecta." for these lamarck proposed the name hexapoda; but that name has been little used, and they have retained to this day the title of the much larger linnaean group, viz. insecta. the position of the arachnida in the great sub-phylum arthropoda, according to recent anatomical and embryological researches, is explained in the article arthropoda. the arachnida form a distinct class or line of descent in the grade euarthropoda, diverging (perhaps in common at the start with the crustacea) from primitive euarthropods, which gave rise also to the separate lines of descent known as the classes diplopoda, crustacea, chilopoda and hexapoda. [illustration: fig. 1.--entosternum, entosternite or plastron of _limulus polyphemus_, latr. dorsal surface. lap, left anterior process. plr, posterior lateral rod or rap, right anterior process. tendon. phn, pharyngeal notch. plp, posterior lateral process. alr, anterior lateial rod or tendon. natural size. (from lankester, _q j. mic. sci._, n s vol. xxiv, 1884)] _limulus an arachnid._--modern views as to the classification and affinities of the arachnida have been determined by the demonstration that _limulus_ and the extinct eurypterines (_pterygotus_, &c.) are arachnida; that is to say, are identical in the structure and relation of so many important parts with _scorpio_, whilst differing in those respects from other arthropoda, that it is impossible to suppose that the identity is due to homoplasy or convergence, and the conclusion must be accepted that the resemblances arise from close genetic relationship. the view that limulus, the king-crab, is an arachnid was maintained as long ago as 1829 by strauss-durckheim (1), on the ground of its possession of an internal cartilaginous sternum--also possessed by the arachnida (see figs. 1, 2, 3, 4, 5 and 6)--and of the similarity of the disposition of the six leg-like appendages around the mouth in the two cases (see figs. 45 and 63). the evidence of the exact equivalence of the segmentation and appendages of limulus and scorpio, and of a number of remarkable points of agreement in structure, was furnished by ray lankester in an article published in 1881 ("limulus an arachnid," _quart. journ. micr. sci._ vol. xxi. n.s.), and in a series of subsequent memoirs, in which the structure of the entosternum, of the coxal glands, of the eyes, of the veno-pericardiac muscles, of the respiratory lamellae, and of other parts, was for the first time described, and in which the new facts discovered were shown uniformly to support the hypothesis that limulus is an arachnid. a list of these memoirs is given at the close of this article (2, 3, 4, 5 and 13). the eurypterines (gigantostraca) were included in the identification, although at that time they were supposed to possess only five pairs of anterior or prosomatic appendages. they have now been shown to possess six pairs (fig. 47), as do limulus and scorpio. [illustration: fig. 2.--ventral surface of the entosternum of _limulus polyphemus_, latr. letters as in fig. 1 with the addition of nf, neural fossa protecting the aggregated ganglia of the central nervous system;