Prof. J. D. Dana on Cephalization. 191 



pair of organs (the posterior), and the feet gain one pair, there 

 being 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 antennae in Spiders is no mark of degradation, since the 

 senses exist in good perfection. 



Descending lower, to the Mijriapods, 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 

 elongation 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- 

 cative coodition 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 Tetra- 

 decapods, two pairs of the mouth-organs are transferred to the 

 locomotive series, making the number of pairs of feet seven, and 

 of mouth-organs four. 



Descending further, to Entomostracans, 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 called) all, 

 for the mouth-organs in this species are all true feet. The Ento- 

 mostracans exemplify decephalization by degeneration {ninth 

 method) — as in the absence of one or two pairs of antennae, 

 the absence of one or two or more posterior pairs of thoracic 

 feet, the absence of the series of abdominal members, and 

 sometimes (as in Limulus) by the reduction of the abdomen to a 

 mere spine. They are degradational forms as well as the Myria- 

 pods ; and hence the apparent difference of grade, which might 

 be supposed to be marked by the number of pairs of mouth- 

 organs transferred backward, cannot serve to subdivide the 

 order. The distinction of the Entomostracans from the higher 

 Crustaceans consists rather in tlieir degradational characters than 

 in any peculiarities of the mouth. In the tribe of Ostracoids 

 [Cypris, &c.) alone, one genus has two pairs of mouth-organs, 

 the rest being legs, another three, and another four, the Tetra- 

 decapod number. 



Ill, Mollusks. — It has been remarked that the subkingdom 

 of Mollusks cannot, from its nature, exempHfy the first method 

 of cephalization. The methods exemplified are the third, fourth, 

 ninth, and tenth. In the transition from the order of Cephalo- 

 pods, the first, to that of Cephalates {Gasteropods), the second, 

 there is a loss of the feet or arms, and a diminished perfection 

 of the senses, and activity is reduced to sluggishness. Descend- 



