2S2 
COMPARATIVE ZOOLOGY. 
ganglia running along the ventral surface of the body 
under the alimentary canal. The brain is in the form of 
a ring encircling the gullet. The alimentary canal and 
the circulatory apparatus are nearly straight tubes lying 
lengthwise — the one through the centre, and the other 
along the back. The skeleton is composed of a horny 
substance (chitine), or of this substance with carbonate of 
lime. All the muscles are striated. 
There are four classes, of which the first is water-breath¬ 
ing, and the others air-breathing. 
Class I.— Crustacea. 
The Crustacea 144 are water-breathing Arthropoda, usu¬ 
ally with two pairs of antennae. 145 Among them are the 
largest, strongest, and most voracious of the subkingdom, 
armed with powerful claws and a hard cuirass bristling 
with spines. Although constructed on a common type, 
Crustaceans exhibit a wonderful diversity of external 
form: contrast, for example, a Barnacle and a Crab. We 
will select the Lobster as illustrative of the entire group. 
A typical Crustacean consists of twenty-one segments, 
of which seven belong to the head, seven to the thorax, 
and seven to the abdomen. 140 In the Lobster, however, 
as in all the higher forms, the joints of the head and tho¬ 
rax are welded together into a single crust, called the 
cejohalo-thorax. On the front of this shield is a pointed 
process, or rostrum; and attached to the last joint of the 
abdomen (the so-called “tail”) is the sole representative 
of a tail—the telson. This skeleton is a mixture of chitine 
and calcareous matter. 147 
On the under-side of the body we find numerous ap¬ 
pendages, feelers, jaws, claws, and legs beneath the cepli- 
alo-thorax, and flat swimmerets under the abdomen. In 
fact, as a rule, every segment carries a pair of movable 
appendages. The seven segments of the head are com- 
