14 
Guide to Crustacea. 
Wall- 
cases 
Nos. 1-3. 
carapace, and the fore part of the body has eight distinct somites 
each bearing a pair of walking legs. In front of these eight 
somites, which form what is called the “ thorax,” is the “ head,” 
a part of the body which is never, in any Crustacean, distinctly 
segmented, but which, since it bears five pairs of appendages, 
must contain at least five somites. The part of the body covered 
by the carapace of the Lobster includes the head and the thorax 
and is known as the “ cephalothorax.” It is necessary to remark, 
however, that the regions of the body named head, thorax, and 
abdomen in the Crustacea are by no means exactly equivalent to 
those so named in the other Arthropoda, for instance in Insects, 
and still less to the parts bearing 
the same names among Vertebrate 
animals. 
This “ segmentation ” of the 
body, or division into somites, is 
not only shown by the external 
covering, but affects some of the 
internal organs as well. Leaving 
these aside for the present, how- 
ever, and considering only the 
exoskeleton, the structure of a 
typical somite will be best un- 
derstood by examining one of the 
separated abdominal somites of the 
Lobster (Fig. 2). This consists of 
a ring of shelly substance, connected 
with the rings in front and behind by areas of thin membrane 
which permit movement in a vertical plane. For convenience 
of description the upper or dorsal part of this ring is called 
the “tergum” (or “tergite”) and the under or ventral part the 
“ sternum ” (or “ sternite ”). To the sternum are attached the 
appendages (or swimmerets), while the tergum overhangs the 
base of the appendage on each side as a flap called the 
“ pleuron.” The terminal segment of the body or “ telson ” never 
bears typical limbs, and on this account and also because of 
its mode of development in the embryo, it is not regarded as a 
true somite. 
The carapace of the Lobster is not formed simply by the terga of 
several adjacent somites becoming soldered together. This is 
shown by a comparison with some of the lower shrimp-like 
Crustacea (Mysidacea, see Table-case No. 5), in which the carapace 
Fig. 2. 
One of the abdominal somites of 
the lobster, with its appendages, 
separated and viewed from in 
front. [Wall-case No. 1.] 
