BEIEF REVIEW OF THE SYSTEM OF LIFE. 439 



Between the Macrurans and the Brachyurans there is the grand distinction that the 

 former are long extended posteriorly, and urosthenic, as regards locomotion (or strong in 

 the posterior extremity, that is, the abdomen), while the latter have short bodies, gathered 

 in closely and compactly behind the cephalic ganglion, and are j>odosthenic, thoracic feet 

 being the only locomotive organs. In rising from the Macruran to the Brachyuran there 

 is a forward transfer in the general structure and in the locomotive function, and thus a 

 great rise in degree of cephalization. 



Under each of these two types of Decapods a wide range of grade is structurally indi- 

 cated, illustrating degrees in cephalization. 



Isopod and Amphipod Tetradecapods. — The Isopods and Amphipods are brachyuran 

 and macruran Tetradecapods, for the series of Tetradecapods is closely parallel with that 

 of the Brachyurans and Macrurans among Decapods. The Isopods have a compact body, 

 a short abdomen, which is not used in locomotion, with relatively short antennse, while 

 the Amphipods have a longer body more loosely put together, usually long antennae, an 

 elongate abdomen, and the abdomen is the chief organ of locomotion — that by which the 

 little animal makes its leaps. Here, again, the lower are the urosthenic and decephalized 

 species, the higher the podosthenic and more cephalized species. 



Entomostracans. — Below the Tetradecopods come the Entomostracans. A part of 

 the Entomostracans are multiplicate species, — the Phyllopods ; and in this character, 

 both in the Entomostracans of Decapod and of Tetradecapod relations, they show out 

 the ancestral worm, and thereby low-grade cephalization. The structure is eminently 

 primitive and was especially characteristic of early Paleozoic Articulate life. 



Besides these there are the simply defective forms among Entomostracans, representa- 

 tive of different stages in embryonic development. Defective forms of similar character 

 occur even among the Macruran Decapods ; for some of the inferior shrimp-like species 

 have one or two of the posterior segments of the thorax without legs, or even wanting ; 

 and in such species (called Schizopods), the thoracic legs have the form characterizing 

 a young stage in development. But among the Entomostracans, the defective stage 

 appears in more extreme forms. The limbs are pai'tly natatory ; the mouth organs are 

 often either pediform or natatory, or of more abnormal forms ; and the abdomen has 

 no appendages except ovarian attached to the basal portion and a caudal pair pertaining 

 to the sixth segment. 



The preceding remarks on the bearing of the principle of cephalization on system and 

 grade in Crustaceans cannot be true for one branch of the Animal Kingdom without hav- 

 ing a wide significance. See, for other examples, Historical Geology, pages 721, 723. 



This subject has much interest in connection with the successional lines in the animal 

 life of the globe which geology has brought to light. But the preceding remarks are not 

 to be understood as intimating anything with regard to the origin of species. There was 

 no such reference in the author's first presentation of the views in 1852.1 j^^ ^i^^t time the 

 idea of evolution by natural causes had scarcely an advocate ; for Darwin's work did not 

 appear until 1859. Neither are the facts now to be regarded as adding to the causes of 

 derivation. This much, however, may be learned from them : — 



1. Whatever the natural causes or methods concerned in evolution, organic conditions 

 have determined lines, limits, and parallel relations, in accordance with the principle 

 of cephalization. 



2. In the evolution of the animal kingdom a " tendency upward " is a necessary con- 

 sequence of the presence of a cephalic nervous ganglion or brain. 



1 Report on Crustacea of the Wilkes Expl. Exped. around the World, 1618 pp., 4to, with a 

 folio Atlas of 96 plates. In the papers on cephalization published in the American Journal 

 of Science, eleven to twelve years later, and subsequently a summary in 1876, the principle 

 of cephalization was illustrated by reference to other classes of animals ; but the speculative 

 conclusions added in those papers are not all in accord with the author's present judgment. 



