290 Kntomostkaca. CRUSTACEA. Phyllopods. 
The Spha:ro.\iians {Sphceromd) live on rocks on 
the sea-coast, and have the faculty of rolling them- 
.selves up in the form of a ball, when touched or 
alarmed. They are all of small size, and several 
species are found in Great Britain. See fig. 190 — 
Sphoerom.n serratum. 
The third group are termed the Sedentary Isopods. 
In this group the sixth pair of false abdominal feet 
are altogether wanting, and their mouth is formed 
more for suction than for mastication ; as the jaws, 
which all the other Malacostraca possess, in these are 
not in e.xistence. They are completely parasitical. 
The Bopyri, or Tail-less Crab-lice, live fixed under 
the vault of the branchial cavity of prawns, shrimps, 
&c., where they may be detected forming a small 
tumour. The males are five or six times smaller than 
the females, and are found attached to their abdomen. 
The figure (fig. 191) is that of another parasitical 
Fig. 191, 
a 
Mud Shrimp louse — lone thoracicus — a Male ; h Female^ 
species, lone thoracicus, which infests the shrimp. 
Sub-class IL— ENTOMOSTRACA. 
The Entomoslraca are for the most part extremely 
small crustaceans, but are very numerous. Their 
external envelope or carapace, which is of a horny or 
coriaceous texture, is formed of one or two pieces which 
either completel}' or in great part cover the body of the 
animal. In some it approaches in appearance so nearly 
to the form of a bivalve shell, that a person who did 
not examine with a microscope the animal contained 
within, would not hesitate at first to call it so. These 
creatui’es are carnivorous, and are very useful in clear- 
ing .stagnant waters of putrid anifnal matter. Their gills 
are attached either to the feet or organs of mastication. 
They are preyed upon by larger animals, and form 
the food of some of our most esteemed fishes. Some 
of them are parasitic, living fixed upon the bodies 
of fishes and other animals that live in the water. 
Many undergo a series of changes, amounting to a 
species of metamorphos.is, in their progress from youth 
to maturity. They are numerous in fresh water, and 
many are marine, those inhabiting the ocean assisting 
materially in producing the luminousness of “ the world 
of waters.” By naturalists they have generally been 
divided into three sections — Branchiopoua, Lophy- 
ROPODA, and PCECILOPODA. 
Section I. — BRANCHIOPODA (Branchiopods). 
The animals belonging to this section of Entomoslraca 
have the mouth furnished with jaws fitted for masticat- 
Fig. 192. 
ing their food. Their branchiae or gills are many and 
attached to the feet, which vary in number, sometimes 
being many and at others few They are in general 
not adapted for locomotion. The antennae are two, or 
four-jointed and ciliated, in some serving as organs of 
motion. They swim freely, and may be observed to 
have their branchial feet in constant motion in the 
water, their action 
being seldom inter- 
rupted, thus venti- 
lating the stagnant 
water in which they 
for the most part 
live, and preventing 
it becoming soon 
putrid. 
They are all ar- 
ranged in two orders, 
Phyllopoda and 
Cladocera. 
Order I.— PHYLLOPODS {Phyllopoda). 
In the animals belonging to this order the feet are 
foliaceous in structure, branchiferous or gill-bearing. 
