'CRUSTACEA. 511 



found parasitic on fish, such as Argulus, Chvndr acanthus, Lernea, 

 and Penella. The seventh and last order contains the strange and 

 curious Acorn shells (Balanus), and Barnacles (Lepas), about which 

 so many romantic and untrue stories have been published in the 

 olden times. 



To commence wjth 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 Caesar 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 



