I SEGMENTATION 5 
of the other internal organs.’ In this respect the Crustacea agree 
with all the other Arthropods, in the adults of which the 
segmentation is confined to the exterior and to the nervous 
system, and does not extend to the body-cavity and its contained 
organs; and for the same reason they differ essentially from all 
other metamerically segmented animals, e.g. Annelids, in which 
the segmentation not only affects the exterior and the nervous 
system, but especially applies to the body-cavity, the musculature, 
the renal, and often the generative organs. The Crustacea also 
resemble the other Arthropoda in the fact that the body-cavity 
contains blood, and is therefore a “haemocoel,” while in the 
Annelids and Vertebrates the segmented body-cavity is distinct 
from the vascular system, and constitutes a true “coelom.” 
To this important distinction, and to its especial application to 
the Crustacea, we will return, but first we may consider more 
narrowly the segmentation of the Crustacea and its main types 
of variation within the group. In order to determine the 
number of segments which compose any particular Crustacean 
we have clearly two criteria: first, the rings or somites of which 
the body is composed, and to each of which a pair of 
limbs must be originally ascribed; and, second, the nervous 
ganglia. 
Around and behind the region of the mouth there is very 
little difficulty in determining the segments of the body, if we 
allow embryology to assist anatomy, but in front of the mouth 
the matter 1s not so easy. 
In the Crustacea the moot point is whether we consider the 
paired eyes and first pair of antennae as true appendages belong- 
ing to two true segments, or whether they are structures swt 
generis, not homologous to the other limbs. With regard to the 
first antennae we are probably safe in assigning them to a true 
body-segment, since in some of the Entomostraca, eg. Apus, 
the nerves which supply them spring, not from the brain as in 
more highly specialised forms, but from the commissures which 
pass round the oesophagus to connect the dorsally lying brain 
to the ventral nerve-cord. The paired eyes are always inner- 
vated from the brain, but the brain, or at least part of it, 1s very 
1 The muscles are to a certain extent segmented in correspondence with the 
limbs ; and the heart, in Phyllopoda and Stomatopoda, may have segmentally 
arranged ostia. 
