COMPAEATIVE MOEPHOLOGT OF THE GALEODID^. 393 



12. The Second Api^endaye. — There can be little doubt that the original form and 

 position of this was leg-like, and that the chelate form was secondarily acquired. 



This has little direct bearing ou the question of the relationship of the Arachnids to Limulus, but it is 

 so far of importance, inasmuch as it is an item in the mass of evidence to show that Scorpio is not 

 a primitive form, so that the detailed comparison between Limulus or Eurypterus and Scorpio, which is 

 one of the foundation-stones in the argument for their close relationship, is of little or no value. 



18. The position and character of these two limbs, together with the consequent 

 specialization of the segments to which they belong, form the most essential feature in 

 Arachnidan morphology. We have in this arrangement a clear adaptation to a special 

 and peculiar method of feeding. The acquisition of a new and successful method of 

 feeding, i. e. of acquiring the materials for growth and reproduction, seems to me the 

 most important of all adaptations for new points of departure in animal morphology. 

 I have already adduced evidence to show that the class Crustacea can be deduced from 

 segmented chaetopod ancestors, which bent their hrst segments ventrally to push the food 

 on which they browsed into their mouths by means of their j)arapodia. From what we have 

 seen in the foregoing, tlie class Arachnida may be deduced from a segmented Annelidan 

 ancestor in which the first two pairs of limbs were shifted forward and grouped round 

 an anterior mouth, which mouth, as a consequence of this very shifting of the limbs,, 

 came to be situated at the end of a beak. The method of feeding which this arrangement 

 implies, that of seizing and wounding prey and sucking their juices from the wound, 

 is common, with slight variations to all the Arachnida. 



That this was the primitive arrangement is certain from the fact that it is not only 

 found in all Ai-achnids, but is accompanied by many different specializations for preventing 

 the escape of the juices, and others agaia for straining them. Further, the different 

 forms of the beak, with the differences in the position of the sucking- ajjparatus, also 

 show that this plan of feeding was the one first adopted by the ancestral form. In 

 this respect the hypothetical ancestor differed from all the other classes of the Arthro- 

 poda. Almost all other Arthropods have chewing-jaws projecting more or less into the 

 oral apertiu'C. It is true that some of these secondarily adopt a method of feeding by 

 sucking, but the adaptation starts from the persistent jaws, which become transformed 

 into lancets, &c., for the purpose. 



Of all Arthropods thus feeding by chewing-jaws, Limulus has reached the extreme, with five pairs 

 of biting-jaws arranged around an oral aperture. The Eurypterids are, if anything, still more highly 

 specialized, inasmuch as the last pair of the Limulus jaws have developed more or less at the expense of 

 those in front of them. The question arises, could Limulus change its method of feeding from biting 

 into sucking juices? It is perhaps possible; but if it did, these jaws woidd persist in some form or 

 other as component parts of the piercing and sucking apparatus, in order to maintain the necessary 

 continuity in feeding. Is it, however, at all likely that the distal eud.s of two pairs of limbs brought 

 prey to a mouth so armed, and crushed it there for the mouth to suck, while the powerful and highly 

 specialized jaws remained useless, and thus atrophied ? This is so improbable that the idea may be 

 dismissed. 



It is true that in Scorpio it is claimed that traces of two pairs of jaws are still to be seen. They form 

 the spoon-like arrangement under the mouth, which prevents the escape of juices. But a comparison 



SECOND SERIES. — ZOOLOGY, VOL. VI, 52 



