254 
CRUSTACEANS. 
of appendages, representing the maxillae and the two following pairs of limbs 
of the adult, have appeared, and the abdominal region has increased in length, 
although, like the greater part of the thorax, is still limbless. A pair of compound 
eyes is present on the sides of the head. After this so-called copepod stage, the 
large eyes become stalked, the abdomen continues to increase in length, and takes 
on the function of swimming, which was before performed by the antennae, and 
the remainder of the thoracic and abdominal limbs appear. Since the thoracic 
limbs are provided with a distinct exopodite, as well as the principal branch or 
endopodite, as in the cleft-footed shrimps (Schizopoda), this larva is known as the 
schizopod stage. Lastly, the median eye and the 
exopodites of the motor-thoracic limbs disappear, and 
the adult form of the Penceus is attained. It is, how¬ 
ever, exceptional amongst the higher forms for the 
young to be set free in the Nauplius stage. The young 
of the lobster, for instance, hatches in the schizopod 
condition; while that of the common crab appears in 
the Zoaea form, although characterised by the presence 
of a long dorsal spine, and a sharp beak on the carapace. 
Moreover, the two pairs of antennae, the mandibles, and 
maxillae, are of small size, while the following two pairs 
of limbs are relatively large, and forked. By means 
of these the minute transparent creature swims, and 
after undergoing several moults passes into a stage termed the Megalopa, which 
is much like the adult, but has enormously large eyes, and swims by flapping its 
long jointed abdomen like a shrimp. 
The Typical Crustaceans,— Subclass Malacostraca. 
Much difference of opinion still obtains as regards the classification of Crus¬ 
taceans, which are here divided into two main subclasses. In the present group, 
comprising the largest and most familiar forms, the number of segments in 
the body is very generally nineteen (but never more), and each has a pair of 
appendages. The first five segments compose the head, which, except in some 
blind species, bears a pair of compound eyes, two pairs of antennae, and three pairs 
of jaws, namely, a pair of mandibles in front, and two pairs of maxillae behind. 
The eight segments behind the head, which constitute the thorax, may be united 
JAWS OF CRAYFISH. 
a, Mandible ; b, c, Maxillae; d, e, f, 1st. 2nd, and 3rd Maxillipedes. 
with the head, as in crabs, when the whole region is termed the cephalothorax, and 
the shield that covers it the carapace. Sometimes too, as in the crayfish, the 
anterior three pairs of thoracic appendages are transformed into jaws, and on this 
