CLASS IV. CRUSTACEA. 
605 
Class IV, CRUSTACEA- 
At the head of this class stand the crabs, lobsters, prawns, and the like ; there are, how- 
ever, raan}^ other animals associated with them of extremely different shapes and habits. In 
general, it may be said of the 
prominent members of the 
class that the form of the 
body is iTSually somewhat 
spindle-shaped, and divided 
into a series of distinct rings, 
articulated together, and al- 
lowing of a considerable 
amoimt of movement. These 
segments are sometimes of 
nearly eqnal size, and are 
furnished with nearly simi- 
lar appendages throughout. 
Sometimes a few of the seg- 
ments acquii'e a greater de- 
gree of development than 
the rest, and the organs of 
motion are confined to these, 
while the appendages of the 
other segments are reduced 
THE COMMON KUEOPEAN CKAB. 
to a more or less rudimentary condition, and in the higher forms the anterior segments become 
fused into a single mass, called the ceplmlothorax, which bears the mouth and organs of motion. 
The skin is generally hardened by a calcareous secretion, constituting a complete cutaneous 
skeleton, within which all the soft parts of the body are inclosed ; the segments are united by a 
thin membrane, which gives flexibility to the whole armor. As the animal has no power of 
adding to the size of this shell to make room for its increasing growth, it casts oif its old coat at 
stated periods, and secretes a new deposit of calcareous matter over its entire surface. The form 
of the articulated appendages — the legs and feet — varies exceedingly. 
The nervous system of the Crustacea always consists of a series of ganglia running along the 
surface of the body, united to each other, and to a cephalic ganglion or brain, by a pair of nerv- 
ous filaments, and giving ofi" nerves to the various organs in their neighborhood. The cephaHc 
ganglion is always situated above the oesophagus, and furnishes nerves to the organs of the senses. 
These are the eyes, the antennse, and in many cases organs of smell and hearing. The digestive 
canal generally exhibits a high degree of development. The respiratory organs consist of 
branchiffi of various forms. With the exception of a single order, the species are all unisexual, 
and reproduction takes place by ova, which are generally attached to the tail of the female for 
some time after exclusion. Their development presents many curious phenomena. 
The immense number and variety of the Crustacea — from the Latin crusta^ a crust, or hard 
covering — have caused them to be divided and subdivided into numerous groups ; we shall, 
however, notice them briefly under the following eleven orders : Decapoda^ Stomapoda, Isopoda, 
Amphipoda, Loemodipoda^ Xyphosura, Phyllop)oda, Ostracoday Copepoda, Parasita, and Cir- 
rhopoda. 
ORDER 1. DECAPODA. 
This term signifies ten-footed, and includes all those stalk-eyed Crustacea, in which the whole 
of the thoracic segments are united with those of the head into a single mass, incased in a com- 
mon shell, with no traces of segmentary division, and which have the branchial organs inclosed 
