6 CRUSTACEA CHAP. 
probably formed of paired trunk-ganghia 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’ 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 Vebalia, 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” that the number of seements 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 foreed 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, eg. 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. Entwick. Mech. ii., 1905, p. 544. 
2 Quart. J. Mier. Sci. xlix., 1906, p. 469. 
