3220 THE JOINTED ANIMALS 



CRABS, LOBSTERS, CRAWFISH, etc. Class Crustacea 



The Crustaceans comprise a large assemblage of Arthropods, presenting great 

 diversity of structure. , Some of the parasitic species have become so simplified in 

 organization that they appear to present no relationship with the higher members 

 of the class, such as crabs, lobsters, wood lice, etc. Yet it is certain that all the 

 species, whether terrestrial or aquatic, free-living, sessile, or parasitic, belong to the 

 same stock, and may be derived from the same fundamental plan of structure. 

 Essentially the body consists of a large number of segments, to each of which is 

 attached a pair of two-branched appendages, the external branch being called the 

 exopodite and the internal the endopodite. Five segments at the front end of the 

 body unite to form a head; the appendages of the first two of these segments being 

 situated in front of the mouth, and performing the office of feelers or antennae, 

 while those of the remaining three segments are transformed into jaws, the first 

 pair of jaws being the mandibles and the following two pairs the maxillae. The 

 rest of the appendages are variously modified, and to some are attached respiratory 

 organs in the form of gills. According to this definition, Crustaceans may be dis- 

 tinguished from the Centipedes, Millipedes, Insects, etc. , by the presence of two 

 pairs instead of one pair of antennae, and by possessing branchial and not tubular 

 (tracheal) respiratory organs. The Arachnida may be separated from Crustaceans 

 by having in front of the mouth only one pair of appendages, acting as jaws and 

 not as antennae, while respiration is effected by means of saccular or tubular 

 ingrowths of the integument. Nor can there be any confusion between Crustaceans 

 and the sea spiders, since the latter have no antennae and all their appendages are 

 placed behind the mouth, which is situated at the extremity of a tubular proboscis. 

 But when we come to the Gigantostraca it is not so simple to point out the differen- 

 tial characteristics of the Crustaceans. It is true that the king crabs are easily 

 distinguishable, and appear to be more nearly related to the Arachnida, yet the 

 Trilobites, which seem to be ancestral forms of the king crabs, show marked 

 affinities to the primitive Crustaceans. 



In a few Crustaceans, especially those leading a terrestrial life, or inhabiting 

 fresh water, the young is very similar to the adult, and gradually attains maturity 



without going through any marked change of form; but 

 in the majority the young upon leaving the egg is unlike 

 the parent, and only acquires its definite form after 

 undergoing a series of molts. The earliest stage, which 

 has been called the Nauplius, is a minute oval body, 

 showing no trace of segmentation, and provided with a 

 single median eye, and three pairs of swimming 

 appendages, which become the two pairs of antennae and 

 the mandibles of the adult. This stage, however, is by 

 no means of invariable occurrence, but is chiefly char- 

 LARVA OF BARNA- acteristic of the lowest members the Entomostraca 

 CI.E (much enlarged). and is rare in the higher Malacostraca. In some mem- 



