CRUSTACEA CHAP. 



probably formed of paired trunk-ganglia which have fused into 

 a common cerebral mass ; and the fact that under certain circum- 

 stances the stalked eye of Decapods when excised with its 

 peripheral ganglion 1 can regenerate in the form of an antenna, 

 is perhaps evidence that the lateral eyes are borne on what were 

 once a pair of true appendages. 



Now, with regard to the segmentation of the body, the 

 Crustacea fall into three categories : the Eutomostraca, in which 

 the number of segments is indefinite; the Malacostraca, in 

 which we may count nineteen segments, exclusive of the terminal 

 piece or telson and omitting the lateral eyes ; and the Leptostraca, 

 including the single recent genus Nebalia, in which the segmen- 

 tation of head and thorax agrees exactly with that of the 

 Malacostraca, but in the abdomen there are two additional 

 segments. 



It has been usually held that the indefinite number of 



segments characteristic of the Entomostraca, and especially the 



indefinitely large number of segments characteristic of such 



Phyllopods as A/ms, preserves the ancestral condition from 



which the definite number found in the Malacostraca has been 



derived ; but recently it has been clearly pointed out by Professor 



Car] tenter- that the number of segments found in the Malacostraca 



and Leptostraca corresponds with extraordinary exactitude to 



the number determined as typical in all the other orders of 



Arthropoda. This . remarkable correspondence (it can hardly 



lie coincidence) seems to point to a common Arthropodan plan 



of segmentation, lying at the very root of the phyletic tree ; 



and if this is so, we are forced to the conclusion that the 



Malacostraca have retained the primitive type of segmentation 



in far greater perfection than the Entomostraca, in some of 



which many segments have been added, c.<j. Phyllopoda, while ( 



in others segments have been suppressed, r.ij. L'ladocera, ' 



Ostracoda. It may be objected to this view of the primitive 



condition of segmentation in the Crustacea that the Trilobites, 



which lor various reasons are regarded as related to the ancestral 



Crustaceans, exhibit an indefinite and often very high number 



of segments ; but, as Professor Carpenter has pointed out, the 



oldest and most primitive of Trilobites, such as Olenelliis, possessed 



1 Herbst, Arch, l-'.n! H-'K-L-. M,i;h. ii., 1905, ]>. f>44. 

 rt. J. J//0-. Sci. xlix., 1906, p. 469. 



