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BULLETIN OF THE BUREAU OF FISHERIES. 
worms, their nearest allies, as well as by the vertebrates. The arthropod possesses in 
addition a dorsal brain, united by a ring-commissure about the esophagus, to a ventral 
chain or “ladder” of paired ganglia, a character also shared by the higher worms; the 
heart is dorsal and overlies the food canal; the cuticle, which encases the body and lines 
every inward fold, is secreted by the outer layer of the skin, the epidermis or hypodermis, 
and is chitinous — that is, contains chitin, a complex nitrogenous substance, by some 
chemists regarded as analogous to cellulose and lignin, which occur typically in plants 
and form the basis of all their woody tissues. This cuticle of the Crustacea is often rein- 
forced by thick deposits of lime and other minerals, thus forming a hard external skeleton, 
to which every peripheral muscle is directly or indirectly attached, and by which every 
soft and delicate organ in the entire body is protected. No other animals possess all the 
several characteristics just enumerated. Since the arthropods embrace the insects, with 
their hundreds of thousands of species, it is not surprising that according to some 
estimates they include three-fourths of all the known species of living animals. 
Of the five commonly recognized classes of arthropods the Crustacea are the lowest 
and most primitive. They fall into two principal subclasses: (a) The Entomostraca, 
embracing all the simpler, more primitive and generally smaller forms, such as water 
fleas, copepods, and barnacles, and (6) the Malacostraca, to which pertain the larger 
and the most highly organized of living Crustacea, such as lobsters, shrimps and crabs. 
The ancient name of the class served the older zoologists to distinguish those animals 
which possessed a “crust,” or a shell flexible at certain joints, from the Testacea, or 
animals like the oyster and clam in which the shelly covering was a hard and unyielding 
“test. ” 
Eight orders of Malacostraca “ have been recognized, of which the more important, 
in view of their size, numbers, economic and general zoological interest, are the Amphi- 
poda and Isopoda, which embrace the beach fleas on the one hand and terrestrial wood 
lice on the other; the primitive Stomatopoda, of which the edible mantis or “praying” 
shrimp are well known representatives, the small Schizopoda, or cleft-feet, and the 
ten-footed and stalk-eyed Decapoda, which mainly interest us. 
In both the isopods and amphipods the eggs are carried in a brood chamber on the 
underside of the thorax, formed by membranous plate-like outgrowths from the thoracic 
legs in the female; the schizopods also carry their eggs in a similar way. 
The breeding habits of the stomatopods are highly peculiar; although celebrated 
for their widely dispersed pelagic larvte, and although it was understood that they 
dwelt in mud burrows under water, and did not carry their eggs attached to the body as 
in decapods, little was known of their early life history until the studies of Professor 
Brooks upon Gonodactylus chiragra of the Bahama Islands appeared in 1893, when he 
gave the first full account of their habits, and the first record of the rearing of a young 
stomatopod from the egg. Fortunately this animal does not deposit its ova deep in the 
mud, but in a burrow, apparently of its own making, in the soft coral rock; they are 
glued together by a viscous cement and molded to fit the convex form of the mother’s 
a In the classification briefly outlined in this chapter we shall follow mainly the excellent work on Crustacea by Geoffrey 
Smith, in vol. iv of the Cambridge Natural History. London, 1909. 
