VI 



A RA CHNOIDEAPHYLOGENY 



539 



concentrated form of the nervous system of the adult animal arises in consequence of 

 the fusing of pairs of ganglia which were separate in the embryo. The whole central 

 nervous system arises in a manner similar to that in other Arthropoda. 



7. The Mesoderm of the Araclmoidea at a certain embryonic stage is developed 

 just as in the Annulata, Protracheata, Antennata, and perhaps also the Crustacea, 

 in the form of 2 lateral segmented streaks or bands with segmental cavities. 



8. The fore- and hind-guts develop in the well-known way as imaginations of 

 the ectoderm (stomodteum and proctodseum). Opinions still differ as to the manner 

 of formation of the tube of the mid-gut. 



9. The first rudiments of the lungs (book-leaf tracheae) appear as imaginations of 

 the ectoderm, and thus in the same way as the tracheae in the Antennata. 



10. Most Arachnoidea when born or hatched from the egg resemble the adult. 

 As far as we know, a post-embryonic metamorphosis occurs only in the Pseudo- 

 scorpionidce and Acarina. The former are hatched in a very imperfect condition, but 



-fte 



FIG. 380. A, B, C, Embryos of Agelena labyrinthica at three stages of development ; in A and 

 B supposed to be spread out flat, in C in the natural form from the venti'al side ; Jd, frontal lobes ; st, 

 stomodseum ; 1-6, 1st to 6th pairs of extremities (of the cephalo-thorax) ; viz. 1, the chelicer* ; 2, the 

 pedipalps ; 3-6, legs ; a, rudiments of abdominal limbs ; aw in C spinning mammillae (after Balfour). 



remain for some time (parasitically) attached to the body of the mother, who carries 

 the eggs about with her. The young larvae of the Acarina (Fig. 361, p. 514) are still 

 devoid of the last pair of extremities, i. e. of the 4th pair of legs. The metamorphosis 

 in the Acarina is often very complicated, and is accompanied by many moults. 

 Sometimes several pupal and larval stages occur. In such cases the metamorphosis 

 is accompanied by the same inner processes as in the Insecta with complete meta- 

 morphosis, i.e. by the breaking up and disappearance of larval organs, and by the forma- 

 tion of the definitive organs out of imaginal portions of the larva. The development 

 and life-history of the Linguatulidce were briefly described in the systematic review. 

 The ontogeny of this group yields no data for deciding their systematic position. 



X. Phylogeny. 



The Arachnoidea form a sharply demarcated natural division of the Arthropoda. 

 Leaving the Linyuatulidce out of consideration, there can be no doubt as to the 



