30 INVERTEBRATE ZOOLOGY 
boundary between the head and the thorax. In the crayfish 
two semicircular, longitudinal grooves extend backward from 
the outer ends of the cervical suture, which separate the sides 
of the carapace from the median, dorsal portion. The sides of 
the carapace are called the branchiostegites; they cover lateral 
folds of the dorsal integument of the animal, which extend 
over the sides of the body and enclose between themselves and 
it the spaces within which lie the gills. These spaces, the gill- 
chambers, thus communicate freely with the surrounding water. 
Pass the handle of a scalpel or other flat object beneath the 
lower edge of the branchiostegite and it will go into the gill- 
chamber. During life a current of water passes constantly 
into the gill-chamber along this lower edge, where it bathes the 
gills and then passes out at the forward end. 
Study the ventral side of the cephalothorax. The most 
important organs here are the appendages. At the anterior 
end of the body are the two pairs of antenne, the longer pair 
being the second. On the lower surface of the basal joint of 
each of the latter is an opening ; these are the external open- 
ings of the kidneys or green glands. Back of the antenne is 
the mouth. It is bounded in front by a lip-like structure called 
the labrum, at the sides by the strong mandibles, and behind. by 
a pair of delicate plate-like projections, called the paragnatha, 
which are not appendages. Press the mandibles aside and 
pass a probe into the mouth. Between the mouth and the 
large claws are five pairs of appendages which assist in the 
act of eating ; they are two pairs of delicate leaf-like maxille, 
just back of the mouth, and three pairs of larger maxillipeds, 
back of them. They are best identified by beginning with 
the hinder pair of maxillipeds, which is just in front of the 
large claws, and working forward, placing a needle or knife 
between the appendages as they are identified. Back of the 
maxillipeds come the large grasping claws or chelipeds, which form 
the principal weapons of offense and defense of the animal, and 
