46 EVOLUTION OF LIFE. 
subject of admiration on the part of naturalists. The Ar- 
ticulata are the most complex in structure of the Inverte- 
brata, or animals without a backbone. The nervous system 
is highly developed, compound eyes are present,the digestive 
system has various parts, excretory glands and ducts have 
been discovered, respiration is carried on by the beautiful 
system of trachea, and of their powers of jumping, flying, 
stinging, biting, and making noises every one is aware. 
The remaining division of the Articulata, including the 
Crabs, etc. (Fig. 51), though differing greatly in shape, size, 
etc., are all alike in their early stages. The Nauplius (Fig. 
50), or primitive stage of every Crustacean, seems to be 
more nearly allied to the Rotatoria than any other group 
of animals. Some of the microscopic forms of the Crus- 
tacea, as Cypris, Daphnia, Cyclops, furnish the transitions 
from the Rotatoria to the Crustacea ; indeed, the Rotatoria 
have been considered as a group of the Crustacea by many 
naturalists. 
MOLLUSCA. 
The most striking difference in the Mollusca, as com- 
pared with the Articulata, is seen in the entire want of that 
segmentation which is so apparent in the Insects or Centi- 
pedes. The body of an Oyster, a molluscous animal, is a 
soft mass, and, though possessed of organs, never exhibits 
the slightest trace of joints, as seen in the higher worms, 
insects, etc. The nervous system is composed of a few 
scattered nervous masses or ganglia, there being no distinct 
chain of ganglia running through the body from head to 
tail. Indeed, some of the Mollusca have no head, being 
known as the Acephala. For this reason the Acephala in- 
clude the Brachiopoda and Conchifera. The Brachiopoda, 
Lamp Shells, or Arm-foot Mollusca, are better called Spiro- 
branchie, as their branchiz, or gills, are arranged in the 
