vi ARACHNOIDEAPHYLOGENY 541 



follow them are either not at all or very little like legs, they are changed into mouth 

 parts. In the Arachnoidea, the appendages which probably correspond with them 

 (the chelicerfe, the pedipalps, and the 1st pair of legs) have preserved far better the 

 character of long-jointed extremities. Now, since we can find no justification in com- 

 parative anatomy or ontogeny for deriving long many-jointed extremities from 

 abbreviated and specialised mouth-parts adapted for chewing, sucking, etc., but are, 

 on the contrary, distinctly justified in assuming that the opposite process is the usual 

 one, we conclude that the racial form of the Arachnoidea branched off from the racial 

 form of the Antennata very early, at a time when the limbs lying directly behind 

 the mouth were not yet changed into specialised mouth-parts. The Arachnoidea 

 (Chelicerota) on the one hand, and the Antennata on the other, would thus represent 

 two branches diverging early from the Tracheate trunk. The Protracheata cannot, 

 it is true, be placed at the root of this trunk, but may still in many points of their 

 organisation much more faithfully retain the primitive condition than do the Arach- 

 noidea and Antennata, and may thus to a certain extent represent an offshoot from 

 the root. 



The above statements must make the relationship of the Arachnoidea and especi- 

 ally of the Scorpionidce with the Xiphosura and Gigantostraca appear at present 

 doubtful. At the same time it cannot be denied that the limbs of the cephalo- 

 thorax in the Arachnoidea show a remarkable agreement with those of the Xiphosura 

 and Gigantostraca, a much greater agreement than with the corresponding limbs of 

 the Antennata. The want of preoral limbs comparable with the antennae is also a 

 point of agreement not to be underestimated. 1 But we may possibly have here only 

 a phenomenon of convergence. The agreement in the rest of the organisation, leaving 

 out of account characteristics common to all Arthropoda, appears to us not so great 

 as to justify a nearer relationship based upon it. Even if the occurrence of rudi- 

 mentary abdominal limbs forces us to assume that the ancestors of the Arachnoidea 

 possessed abdominal limbs, the -'same is true', of the Hexapoda also, the Myriapoda 

 still possessing limbs on all the trunk segments. 



The comparison of the book-leaf tracheae with the book-like gills of the Xiphosura 

 seems far-fetched compared with their derivation from the tufted tracheae. The 

 assumption that the tubular tracheae in the Arachnoida have arisen independently of 

 those of the Protracheata and Antennata can only be resorted to as a makeshift. Mal- 

 pighian vessels are wanting in the Xiphosura. The sexual organs may emerge at very 

 different regions of the body in the Antennata, as \vas seen as early as in the Myria- 

 poda, and therefore no very great weight should be attached to the circumstance that 

 their position is almost similar in the Arachnoidea and the Xiphosura. The presence of 

 coxal glands, which emerge in the Arachnoidea and Xiphosura on the third pair of 

 legs, does not bear much upon this question, since, on the one hand, coxal glands may 

 occur on other pairs of legs as well in the Arachnoidea, and on the other, these glands 

 are very widely distributed among the Protracheata and Antennata (especially 

 Myriapoda}, and apparently were originally found in all the pairs of legs, as is .still 

 the case in the Protracheata. 



In any case further investigations as to the relations between the Arachnoidea and 

 the Xiphosura cannot but be fruitful, and may throw much light upon the as yet 

 by no means solved problem of the relationship of the two groups. 



The Pantopoda (Pycnogonidce) are also often considered as related to the Arach- 

 noidea, a view which was arrived at originally in consequence of the great similarity 

 in appearance of the two groups. This view had, however, to be abandoned when 

 their organisations were more closely compared (cf. p. 424). 



1 See footnote on page 516. 



