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 
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 distinguished 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 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 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¬ 
acteristic of the lowest members—the Entomostraca— 
and is rare in the higher Malacostraca. In some members 
of the latter group, nevertheless, it does occur, as in one 
In this the Nauplius passes into a stage called the Zocea, in which four pairs 
NAUPLIUS LARVA OF BARNACLE 
(much enlarged). 
of the shrimps ( Pencvus ). 
