If the above considerations are correct, the appendages of the Arachnida retain in many respects a very much more primitive condition than those of Insects. In the first place, both the cheliceræ and pedipalpi are much less differentiated than the mandibles and first pair of maxillæ with which they correspond. In the second place, the first pair of ambulatory limbs must be equivalent to the second pair of maxillæ of Insects, which, for reasons stated above, were probably originally ambulatory. It seems in fact a necessary deduction from the arguments stated that the ancestors of the present Insecta and Arachnida must have diverged from a common stem of the Tracheata at a time when the second pair of maxillæ were still ambulatory in function.

With reference to the order of the development of the appendages and segments, very considerable differences are noticeable in the different Arachnoid types. This fact alone appears to me to be sufficient to prove that the order of appearance of the appendages is often a matter of embryonic convenience, without any deep morphological significance. In Scorpio the segments develop successively, except perhaps the first post-oral, which is developed after some of the succeeded segments have been formed. In Spiders the segment of the cheliceræ, and probably also of the pedipalpi, appears later than the next three or four. In both these types the segments arise before the appendages, but the reverse appears to be the case in Chelifer. The permanent appendages, except the cheliceræ, appear simultaneously in Scorpions and Spiders. The second pair appears long before the others in Chelifer, then the third, next the first, and finally the three hindermost.

Bibliography.

Scorpionidæ.

(434) El. Metschnikoff. “Embryologie des Scorpions.” Zeit. f. wiss. Zool. Bd. XXI. 1870.
(435) H. Rathke. Reisebemerkungen aus Taurien (Scorpio). Leipzig, 1837.

Pseudoscorpionidæ.

(436) El. Metschnikoff. “Entwicklungsgeschichte d. Chelifer.” Zeit. f. wiss. Zool., Bd. XXI. 1870.
(437) A. Stecker. “Entwicklung der Chthonius-Eier im Mutterleibe und die Bildung des Blastoderms.” Sitzung. königl. böhmisch. Gesellschaft Wissensch., 1876, 3. Heft, and Annal. and Mag. Nat. History, 1876, XVIII. 197.

Phalangidæ.

(438) M. Balbiani. “Mémoire sur le développement des Phalangides.” Ann. Scien. Nat. Series V. Vol. XVI. 1872.

Araneina.