53° 



CRUSTACEA— ORDER PHVLLOPODA. 



GLASS I.— CRUSTACEA. 



The Crustacea may be considered as tlie marine representatives of the Insects; 

 for although a few insects are marine, and some Crustacea are found on land 

 or in fresh water, yet an overwhelming preponderance of the Crustacea are 

 exclusively marine. In the perfect state they are covered with a hard jointed 

 shell. Sometimes the head is united with the thorax, as in the class Aradi- 

 nida, but it is more often separated. Being water animals, 

 Larval Forms of or living with few exceptions in damp places on land, they 

 Crustacea. breathe with gills. They have generally two eyes, two pairs 

 of antennse, three pairs of jaws, three pairs of foot-jaws, 

 the two outer pairs of which often serve the purpose of legs, and five 

 pairs of legs. Most of the species undergo a peculiar metamorphosis, like 

 that of the Barnacles, which has led to the latter 

 being now classed as retrograde Crustacea. These 

 larval forms are generally called Navplius, or Zom, 

 these names having been given to them when they 

 were first discovered, and were supposed to be perfect 

 organisms. They have large eyes, and curious bifid 

 appendages terminating in long bristles, and utilised 

 as swimming feet. The Natiplius-iona (compare -Kji. 

 5) shown by many Crustacea on emerging from the 

 egg, is oval, with a single eye, and it gradually acquires 

 three pairs of limbs. In the Copepoda and Ostracoda 

 it passes gradually into the perfect state by successive 

 moults; but in the Siphoiiostoma it undergoes a re- 

 trograde metamorphosis, losing the eyes, and limbs, 

 as locomotive organs. This form of larva is seldom met 

 with among the Decapoda. The Zoea-form of larva 

 has seven pairs of jointed appendages, of which the 

 foot- jaws are very large ; and two large eyes. (See 

 Fig. 1.) When the Zoea was first discovered, it 

 was thought to be a creature of the greatest rarity and interest, and it is 

 amusing to read in the old books of the capture of single specimens of a 

 Zoea at intervals of years, in distant parts of the globe. Now they are 

 known to be simply the larval forms of many of our commonest Omstmea. 

 Similar unexpected discoveries perhaps still exist, in the case of other, 

 as yet unsuspected, inhabitants of our globe. 



Crustacea are carnivorous or omnivorous, and some of the smaller kinds 

 are parasitic. Some Cr^istacea are of very small size, while others are the 

 largest of known Arthropoda. Their average size is considerably above that 

 of insects. There are probably no species which can be considered actually 

 injurious to man ; but many are of considerable economic importance. 

 Crabs, shrimps, lobsters, prawns, and crayfish are the only Arthropoda which 

 form staple articles of diet among civilised nations at the present day, though 

 there is no reason why locusts and large wood-feeding grubs 

 Edible and caterpillars should not be eaten. Locusts have always 



Arthropoda. been relished in the East; and the Cossus, which was pro- 

 bably the larva of some large beetle, was considered a damty 

 by the Romans. However, with the exception of cheese-maggots and cheese- 

 mites, both of which are probably less wholesome than the insects just 



Fig. 1. — Zoea-Larva of 



Spider-Crab (Maia 

 Sqitiiuido). Magnified. 



