THE FORM OF THE LARVA. 159 



Arachnid a arose from forms nearly related to the Xiphosura, for 

 the Arachnida agree with the Xiphosura in many more points 

 than with the Pantopoda. If we must remove the union of the 

 two to such a remote period, the few points of comparison again 

 lose their significance, seeing that they refer chiefly to the more 

 highly developed forms and not to the lower forms. To derive 

 the Pantopoda directly from the Arachnida, however, seems im- 

 possible, the latter having attained far too high a grade of 

 organisation to allow of such a derivation. 



Even if the Pantopoda were originally related to the Arachnida 

 or some other segmented form, they have in their whole organisation 

 become far removed from it, and have become markedly specialised. 

 The decided preponderance of the limbs over the trunk, and the 

 almost complete degeneration of the latter (Fig. 74) determined 

 the displacement of the internal organs (intestinal caeca and genital 

 glands) into the limbs. The opening of the genital organ on the 

 second joint of the limbs is probably a consequence of this change, 

 and thus has a secondary character. In those cases in which the 

 genital apertures are found, not on several limbs, but only on the 

 seventh pair, as in Pycnogonum, we might be inclined to derive this 

 condition from that in Limulus and the Arachnida, in which the 

 genital apertures lie in the first [second] abdominal segment, and to 

 regard it as primitive, but such an assumption is not supported 

 by any convincing evidence. 



The reduction of the trunk as compared with the limbs becomes 

 still more marked through the degeneration of the abdomen. The 

 latter is merely a short, truncated appendage of the body (Fig. 74), 

 but the presence of two pairs of ganglia (Dohrn) shows that it 

 originally consisted of more than one segment. In Ammothea and 

 Zetes, the abdomen shows externally a division into two parts, 

 and in some other Pantopoda evidence of a larger number of 

 segments (three to seven) is said to be forthcoming (Hoek, Xo. 7, 

 pp. 453 and 454). 



Should the Pantopoda prove to be connected at the roots with the 

 Arachnid stock, they would thus in a certain way be related to 

 the Crustacea. The latter, however, appear to us to be too far 

 removed in structure to admit of any relation between the Panto- 

 podan larva and the Nauplius. Those recent observers who have 

 most thoroughly studied the ontogeny of the Pantopoda cannot find 

 any close relation between the two. Hoek regards the larva as 

 representing the primitive racial form, just as the Nauplius was 



