COMPARATIVE MORPHOLOGY OF THE GALEODID.E. 403> 



Helation of the Hypothetical Form to other Artliropoda. — This important subject 

 can only be briefly touched upon. I may perhaps be allowed to suggest a few considera- 

 tions which I think may serve to elucidate the different origins oi' the Arthropoda. 



It seems to me that the contrast above described (p. 388, PI. XXXIV. tig. 17) between 

 the primitive segmentation of the Arachnida on the one hand, and of the Trilobites and 

 primitive Crustacea on the other, in adaptation to two different and directly opposite 

 methods of feeding, suggests that the Hexapods and Myriapods may be intermediate 

 specializations. In these last-uamed Artlwopods, the first pair* of limbs, like the chelicerse 

 of Arachnids, moved upward and forward, but, as sensory feelers, they did not require the 

 powerful muscular attachment necessary for prehensile limbs : hence there wei*e no 

 cephalic lobes. The next two pairs of limbs were specialized as chewing-jaws within the 

 mouth-aperture, while the fourth pair might either be free to do the same or fuse with 

 the under-lip to form a compound labium. The anterior position of the mouth would 

 render it not easy for many pairs of limbs to function as jaws ; in order to enable even 

 two or three pairs to do so, the anterior segments have had to be so compressed, that 

 a distinct region, the head, has been formed. 



We thus have the Hexapods with an initial specialization of segments differing from 

 that either of the Arachnids or of the Crustacea. Whereas in the former there was a 

 backward and dorsal distortion of the 1st segment, and in the latter a ventral bending 

 of the 1st segment, in tlie Hexapods the first four segments were merely longitudinally 

 compressed in order to allow the limbs of the 2nd, 3rd, and 4th segments to function as 

 jaws round an anterior mouth-aperture. This applies also to the Myriapods, but in them 

 we find a varying number and different arrangement of limbs functioning as jaws. 

 This difference between the Myriapods would thus separate them entirely from the 

 Hexapods and also from one another. 



One other difference between the Arachnids, the Crustacea, and the Hexapoda appears 

 to me to be fundamental. If my deduction of tracheal invaginations from acicular 

 glands prove correct it would suggest different origins for the limbs in these three 

 derivatives of the Chsetopoda. 



I would deduce the Crustacean limb from the Annelidan paraj)odinm by ventral 

 displacement, so that the ventral jmrapodia became the chewing-jaws, the dorsal the 

 swimming (and ultimately walking) limbs, which carried cirri (exopodites) and gills 

 (ej)ipodites). The dorsal portion of the limbs probably long continued as swimming- 

 plates, during which time their special groups of sette were dispersed, wliile the supporting 

 aciculae of the original parapodia were lost (see, however, footnote, p. 380). On the ventral 

 portion of the limbs the seta? may have furnished the bristle-like teeth and other hairs. 

 (PI. XXXIV. fig. 18, A.) 



The ancestors of the Arachnids, like those of the Crustacea, ajipear to have developed 

 the dorsal parapodia into the walking-legs. But these limbs probably passed through no 

 flat swimming stage. The ventral parapodium disappeared or became merged in the 

 coxal joint of the more developed limb. The acicular gland of the dorsal branch as it 

 became a long limb degenerated, but that of the ventral and less specialized branch 

 persisted as the tracheal invagination (PI. XXXIV. fig. 18, B). This figure is not 



63* 



