1921] Schmitt: The Marine Decapod Crustacea of California 13} 
Il. EXPLANATION OF TERMS AND MEASUREMENTS 
The Decapoda are that order of Crustacea in which the carapace 
fuses dorsally with all the thoracic somites and in which the three 
anterior pairs of thoracic appendages are turned forward to act as 
maxillipeds, or foot-jaws. Their gills are contained within a special 
branchial chamber on each side of the thorax but underneath the cara- 
pace. 
The order Decapoda includes the forms commonly known as erabs, 
shrimps, prawns, crayfish, lobsters, and hermit crabs. 
The typieal deecapod crustacean is divided into three major body 
regions, head, thorax and abdomen, the first two of which are coalesced, 
and are dorsally covered by a common, unjointed, cephalo-thoracic 
shield, the carapace. Typically there are nineteen pairs of append- 
ages, a pair to each of the nineteen true somites into which the body 
is divided, grouped as follows: 
I. The Head, composed of five indistinguishably fused somites, cov- 
ered by the anterior portion of the carapace, and carrying 
the first five pairs of appendages: 
1. The first pair of appendages are the antennules, or first 
antennae as they are sometimes known. 
2. The second pair of appendages are the antennae. 
3. The third, the mandibles, or jaws proper. 
4 and 5. The fourth and fifth pairs are the first and second 
pairs of maxillae, or accessory jaws. 
In view of their development the eyes have not been con- 
sidered true appendages here. Whether they are or not is a 
much mooted question (ef. Calman, 1909, p. 9). 
II. The Thorax, composed of eight somites which are dorsally fused 
with and also covered by the carapace, and which carry the 
next eight pairs of appendages: 
6, 7 and 8. The first three pairs of thoracic appendages are the 
first, the second, and the third, or outer, external, pair 
of maxillipeds, foot-jaws. 
9. The fourth pair of thoracic appendages is the first of the 
five pairs of legs from which this order of the Crustacea 
derives its name. The legs of this first pair are usually 
chelate, though they may be either subchelate or simple, 
