+ _ J. D. Dana on Cephalization. 
appendages. 
Passing down from Insects to Spiders, the mouth loses one 
owe of organs, the posterior, and the feet gain one pair, there 
ing four pairs of feet in Spiders—that is, there is a transfer of 
one pair from the cephalic to the locomotive series. The absence 
of antennz in Spiders is no mark of degradation, since the senses 
exist in good perfection. 
Descending lower, to the Myriapods, the Articulate type passes 
below the range of normal variation into a degradational form, 
and one which, like that of Worms, admits of indefinite posterior 
ngation or multiplication of segments (by the eighth method 
of decephalization), and hence it has no closed or fixed limits, 
like that of Spiders or Insects. Under this loose and multipli- 
eative condition of the system, there is no regular transfer back- 
ward of another pair of mouth-organs: the type is distinguished 
instead, by the degradational character just mentioned. 
_ 2. The facts among Crustaceans have already been pointed out: 
that, descending from Decapods, (Crabs and Lobsters,) which have 
six pairs of mouth-organs and five of feet, to Tetradecapods, two 
pairs of the mouth-organs are transferred to the locomotive s& 
ries, making the number of pairs of feet seven, and of mouth- 
organs four. 
escending further, to Hntomostracans, or the third order, the 
mouth-organs lose one or more of the remaining pairs, and some- 
times, as in Limulus (or the Horse-shoe Crab, as it is ealled) all, 
for the mouth-organs in this species are all true feet. The kn- 
tom¢ exemplify decephalization by degeneration (ninth 
method): as in the absence of one or two pairs of antenn®; — 
the absence of one or two or more posterior pairs of thoraci¢ — 
the series of abdominal members; 22 
Limulus, by the reduction of the abdomen toa — 
mere spine. They are degradational forms, as well as the Myra — 
ence, the apparent difference of grade, which might — 
rder. The distinction of the Entomostracans from the higher E 
