Prof. J. D. Dana on Cephalization. 189 



the coalescence of the head and thorax^ or of these with the ab- 

 domen; the extension of the gastric viscera towards, or into, 

 the head. 



10. By excessive size of body through mere vegetative en- 

 largement, — as in the Megatherium, the female Bopyrus, Limu- 

 lus, &c. 



Degradation, or a decline helow the normal level, may hence 

 be — 



I. Multiplicative. Methods 7, 8, above. 



11. Degenerative. Methods 3, 4, 9. 



III. Vegetative. Method 10. Also IV. Phytoid (or plant- 

 like), when animals (as Polyps) have (11) the power of budding, 

 or (12) a radiate structure, or (13) attachment below; and in 

 such cases the decephalization is often almost as complete as in 

 plants*. 



Examples of cephalization by the first method, or by a transfer 

 of members from the locomotive to the cephalic series (or of 

 decephalization by the reverse), occur in the two highest sub- 

 kingdoms, those of Vertebrates and Articulates. They fail in the 

 two lower subkingdoms, those of Mollusks and Radiates, because 

 of the absence of the necessary structure for showing it. 



The examples under Vertebrates and Articulates, and the rela- 

 tions of the orders among Mollusks, may be briefly considered. 



I. Vertebrates. — Only a single example in the class of Mam- 

 mals, or even in the whole subkingdom of Vertebrates, is possi- 

 ble, owing to the fixed nature and simplicity of the head, and 

 also the limited number of feet, two pairs being the maximum. 



This one example has already been pointed out and shown to 

 be the basis of the grand distinction between Man and other 

 Mammals. In passing downward from the exalted position 

 which Man holds, there is a transfer of the fore limbs to the 

 locomotive series : the structure of the head in Vertebrates, even 



* The methods of decephalization in Crustaceans are embraced under 

 two heads, by the writer, in his paper on the Classification of Crustaceans 

 (Silliman's Journ. ser. 2. vol. xxii. p. 28, and Expl. Exp. Rep. on Crustacea, 

 p. 1412), as follows:— 



"1. A diminution of centralization, leading to an enlargement of the 

 circumference or sphere of growth at the expense of concentration, as in 

 the elongation of the antennae and a transfer of the maxillipeds to the 

 foot-series, the elongation of the abdomen and abdominal appendages, &c. 



" 2. A diminution of force as compared with the size of the structure, 

 leading to an abbreviation or obsolescence of some circumferential organs, 

 as the posterior thoracic legs or anterior antennae, or the abdominal ap- 

 pendages (where such appendages exist in the secondary type embracing 

 the species). 



" These circumstances, moreover, are independent of a degradation of 

 intelligence by an extension of the sphere of growth beyond the proper 

 limits of the sphere of activity." 



