CLASS IV. CRUSTACEA. 



605 



THE COMMON EUEOPEAN CEAB. 



Class IV. CRUSTACEA. 



At tlie head of this class stand the crabs, lobsters, prawns, and the like ; there are, how- 

 ever, many 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 usually somewhat 

 spindle-shaped, and divided 

 into a series of distinct rings, 

 articulated together, and al- 

 lowing of a considerable 

 amount of movement. These 

 segments are sometimes of 

 ncaiiy e(|ual size, and are 

 furnished with nearly simi- 

 lar appendages throughout. 

 Sometimes a few of the seg- 

 ments acquire 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 

 to a more or less rudimentary condition, and in the higher foi-ms the anterior segments become 

 fused into a single mass, called the cephalothorax, which bears the mouth and organs of motion. 

 The sldn 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 off 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 oflF nerves to the various organs in their neighborhood. The cephalic 

 ganglion is always situated above the oesophagus, and furnishes nerves to the organs of the senses. 

 These are the eyes, the antenna?, and in many cases organs of smell and hearing. The digestive 

 canal generally exhibits a high degree of development. The respiratory organs consist of 

 branchite 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 : Deccqxida, Stomapoda, Isopoda, 

 Ampliipoda^ Lcemodlpioda^ Xyphosura^ Phyllopjoda^ Ostracoda, Copepoda^ Farasita, and Cir- 

 rhopoda. 



ORDER L 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 



