194 Prof. J. D. Dana on Cephalization. 



organs) of a Crab. But in Limulus there is extreme of degrada- 

 tion^ all the members being large and stout feet^ only the basal 

 joints of the feet serving as jaws, — the body being enormously 

 enlarged by mere vegetative growth, — the antennae wanting, or 

 reduced to a pair of pincers, and the animal sluggish, a sport of 

 the waves on the beach ; while in Insects there is extreme of 

 cephalization, the pairs of feet only three and those small and 

 slender, and the body minute in comparison — the antennae well 

 developed, and serving as delicate organs of sense — the animal 

 active, and wonderful in its instinctive habits and knowledge. 



The parallelism above shown between Insecteans and Crusta- 

 ceans proves that Insects, Spiders, and Myriapods are orders in 

 a single class, and not separate classes*. Moreover the orders 

 under the classes of Insecteans and Crustaceans constitute par- 

 allel series, the first two of each being closed types, within the 

 range of normal variation, and the last one of each {Myriapods 

 and Entomostracans) being a degradational type, though different 

 one from the other in kind of degradation. The parallelism 

 between the series would be well exhibited if the orders were 

 thus named : — 



Those of Insecteans, (1) Hexapods, (2) Octapods, (3) Myria- 

 pods. 



Those of Crustaceans (1) Decapods, (2) Tetradecapods, (3) Co- 

 lopods, this last term (from k6Xo<; and 7rov<;) signifying defective 

 feet or members, which is the prominent characteristic of the 

 order. 



The parallelism extends even further than has been men- 

 tioned. The Tetradecapods are not an intermediate type be- 

 tween Decapods and Entomostracans ; on the contrary, they lie 

 quite out of the range of either. The Decapods, in their de- 

 gradational species, pass almost into Entomostracan forms, and 

 not into Tetradecapod forms. So among Insecteans, the Spiders 

 have the same isolated position and defined limits. Insects, in 



* The grand distinction of the subdivision of Insects consists in their 

 having three pairs of mouth-organs and three pairs of feet ; of Spiders, in 

 having two pairs of mouth-organs and four pairs of feet ; of Myriapods, in 

 having, through degradation, an indefinite number of segments and feet. 

 Hence, to include Spiders, Myriapods, and the Hexapod group of Pulices, 

 LepismcB, Pediculi, and the like, in one division called Aptera, as is done 

 by some naturalists who adopt the general division of Insecteans, is a 

 violation of all true affinities. 



Professor Agassiz recognizes the same three classes of Articulates as 

 above by the writer, and the same subdivisions, or orders, of Insecteans, 

 but " from embryological data." The writer has not felt ready to deprive 

 Spiders and Myriapods of their place in separate classes, co-ordinate with 

 those of Insects, Crustaceans, aud Worms (a common method among 

 zoologists), until recently, when the special application to these Articulates 

 of the principle above explained occurred to him. 



