CRUSTACEA 



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 Entomostraca, 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 Apus, 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 

 Carpenter 2 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 

 be 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, e.g. Phyllopoda, while 

 in others segments have been suppressed, e.g. Cladocera, 

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

 condition of segmentation in the Crustacea that the Trilobites, 

 which for 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 Olenellus, possessed 



1 Herbst, Arch. Entivick. Mcch. ii., 1905, p. 544. 

 2 Quart. J. Micr. Sci. xlix., 1906, p. 469. 



