CRUSTACEA, SIL 
found parasitic on fish, such as Argulus, Chondracanthus, Lernea, 
and enella. The seventh and last order contains the strange and. 
curious Acorn shells (Ba/anus), and Barnacles (Lefas), about which 
so many romantic and untrue stories have been published in the 
olden times. 
To commence with the best known of these orders, the Decapoda, 
we find it containing the crabs and lobsters ; these may be regarded 
as the chiefs or lords of the Crustacea. The crabs have very large 
claws, and often smooth backs ; the lobsters have also large claws 
and the back sometimes covered with spines. Tiberius Cesar had 
the face of a poor fisherman scratched by the rugged shell of a 
craw-fish, 
Both crabs and lobsters are amazingly fecund, and lay an im- 
mense number of eggs, each female producing from 12,000 to 20,000 
in the season. These eggs are, in the lobster, arranged in packets, 
which are attached to the appendages of the lower surface of the tail, 
to which they are connected by a viscous substance. The manner in 
which the female lobster disposes of her burden is curious and 
interesting. Whether she bends or stands erect she is able to hold it 
concealed or exposed to the light at will. Sometimes, according to 
Coste, the eggs are left immovable, or simply submerged ; at others 
they are subjected to constant agitation by gently moving the false 
feet which contain them to and fro. When first extruded from the 
ovary the eggs are very small, but they seem to increase during the 
time they are borne about under the tail, and before they are com- 
mitted to the sand or water they have attained the size of small shot. 
The evolution of the germ is in progress during six months. “As the 
young lie enclosed within the membrane of the egg,” says Couch, “ the 
claws are folded on each other, and the tail is flexed on them as far 
as the margin of the shield. The dorsal spine is bent backwards, and 
lies in contact with the dorsal shield, for the young when it escapes 
from the egg is quite soft, but it rapidly hardens and solidifies by the 
deposition of calcareous matter on what may be called its skin.” 
As soon as they are born the young Crustaceans withdraw from 
the mother and ascend to the surface of the water in order to gain 
the open sea. They swim in a circle; but their pelagic life is not 
of long duration ; they quit it after their fourth moult, which takes 
place between the thirtieth and fortieth day, at which time they lose 
the transitory organs of natation which they have hitherto possessed. 
After this they are no longer able to maintain themselves on the 
surface, but drop to the bottom. Henceforth they are condemned 
to remain there, and such walking as they can exercise becomes their 
' 
