Page:The Eurypterida of New York Volume 1.pdf/144

 stages of the eurypterids are described in detail now for the first time, it appears very appropriate to test the hypothesis of this arachnidan relationship by the ontogenetic evidence now available. Kingsley [1893, p. 228] cites, in his full discussion of the agreements and differences between Limulus and the crustaceans on one hand and the arachnids on the other, these six points of agreement:


 * 1) A branchial respiration
 * 2) The possession of biramous appendages
 * 3) The absence of malpighian tubes
 * 4) The absence of salivary glands
 * 5) The absence of embryonic envelops
 * 6) The presence of compound eyes

and 28 points in which Limulus and the arachnids agree, and in which both differ from the other "Tracheates" (Hexapoda and Myriapoda). The following of these points are considered as of special importance for the association of Merostomata and Arachnida:

(1) The numerical homologies of segments and appendages; (2) the exact homologies existing in the respiratory organs; (3) the fact that the cephalothoracic appendages are pediform, the basal joints serving as jaws; (4) the presence of true nephridia opening in the base of the third or fifth pair of appendages or in both; (5) genital openings in the seventh (or more probably eighth) segment of the body; (6) extreme length of the midgut; (7) presence of an internal structure, the entosternite; (8) inclusion of the ventral nerve cord and its nerves in the external artery and its branches; (9) the close similarities in the central nervous system.

The last chapter of Thorell's paper on A Silurian Scorpion from Gotland is entitled: Are the Merostomes Arachnids? Here the author gives a critical discussion of these points of resemblance and concludes that the characters on the basis of which the merostomes have been considered as nearly related to the scorpions are "fully counterbalanced by the differences exhibited in the structure of the organs of respiration and by the fact that the scorpions and all the higher Arachnida are provided with