192 Prof. J.'D. Dana on Cephalization. 



ing to the third order, or Acephals, the antennae fail, the eyes be- 

 come imperfect or obsolete, locomotion becomes very imperfect, 

 and in some fails altogether. Among Bryozoans, a still inferior 

 order, all the organs of the senses fail, and there is the radiate 

 structure of vegetation as well as its sessile character. 



The difference in cephalization between an oyster and a clam 

 is very strongly marked, — the oyster, when placed in its normal 

 position, having its body nearly all posterior to the beak, being 

 merely a large gastric mass ; and the clam having one-third of 

 the body anterior to the beak, and really exhibiting something 

 stately in mien compared with the oyster. 



Other illustrations of the subject might be given ; but they 

 are not necessary to explain the general principle in view. 



The number of pairs of feet in the subkingdoms of Vertebrates 

 and Articulates, under those types which afford examples of the 

 first method of cephalization, is as follows : — 



I. Vertebrates. 

 1 in Man ; 3 in all other Vertebrates. 



II. Articulates. 



1. Under Insedeans. 3 in Insects; 4 in Spiders. 



3. Under Crustaceans. 5 in Decapods ; 7 in Tetradecapods. 



The number of pairs of feet in the different groups are then 

 1, 2, 3, 4, 5, 7. Only one case of typical transfer occurs in each of 

 the three classes illustrating the subject — Mammals, Insecteans, 

 and Crustaceans ; and these cases occur uniformly between the 

 two highest orders of the class. 



Man^s title to the place assigned him in our former paper 

 appears therefore to be unquestionable. 



The types of Vertebrates and Articulates do not admit of 

 any homological comparisons. 



The types of Insecteans and Crustaceans are modifications of 

 a common type ; yet the two are so widely different, that it is 

 far from true that the five pairs in the highest Crustaceans cor- 

 respond to the four in Spiders ^j/ws a preceding pair of mouth- 

 organs. The head and locomotive part of the thorax in the 

 Land-Articulates appear to correspond unitedly, as stated by 

 Latreille, to the cephalic portion of the Crab, — that is, to nine 

 anterior segments out of the fourteen cephalothoracic. In other 

 words, this part of the body of an Insect is an extreme con- 

 centration of the anterior portion of a Crustacean — an example 

 of extreme cephalization; while a Crustacean is a diluted 



