CRUSTACEANS. 253 



Crabs, Lobsters, Crayfish, 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 

 organisation 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 

 antennas, 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 distinguished from the Centipedes, Millipedes, Insects, etc., by the presence 

 of two pairs instead of one pair of antenna?, 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 antennas, 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 differential characters 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 moults. The earliest stage, which 

 has been called the Nav/plius, 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 antennas and 

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

 no means of invariable occurrence, but is chiefly char- 

 acteristic of the lowest members — the Entomostraca — (much enlarged), 

 and is rare in the higher Malacostraca. In some members 



of the latter group, nevertheless, it does occur, as in one of the shrimps ( Pencews). 

 In this the Nauplius passes into a stage called the Zocea, in which four pairs 



