20 SKELETAL ORGANS. 
running direct from sense-organs to muscles (or the motor 
organs). In higher types, there appear nerve-cells with 
connecting nerve-fibres, and the nerve-cells become aggre- 
gated into masses called ganglia. The nerves become 
differentiated into afferent (sensory) nerves, or those which 
carry impulses to the ganglia, and efferent (motor) nerves, 
which carry impulses from the ganglia to the muscles. 
The brain is a specially differentiated mass of nerve-cells 
often composed of several ganglia aggregated together. It 
is usually at the anterior end in close contiguity to the 
main sense-organs. 
Skeletal System.—The skeletal system consists of 
certain parts of the body which are formed by the secretory 
activity of the protoplasm. These may be of three 
principal kinds according to the material of which they 
are composed :— 
1. In a number of the lowest types seca is employed 
in the formation of a skeletal system, but this substance is 
confined to the Protozoa and Celenterata. 
2. Calcareous matter is a very common skeletal material. 
It occurs throughout the animal phyla, and is specially 
important in the Vertebrata in which it enters into the 
constitution of bone. 
3. Horny matter or eratin is also very widespread. 
Keratin is a complex nitrogenous chemical substance, thus 
differing from the two former materials. Keratin, or its allies, 
forms the main constituent of cuticles, horns, nails, hair, 
hoofs, &c. os 
Morphologically, these various skeletons may be divided 
into exoskeletons and endoskeletons. The exoskeleton is 
formed on the outside of the body and belongs to the 
so-called zxztegumentary system. The endoskeleton is pro- 
duced in the deeper tissues, usually in the middle layer or 
mesoderm. 
Lastly, a more or less consistent skeleton is composed 
of certain modified tissues, such as cartilage or connective 
tissue (see Chapter IV.). 
It is not uncommon for many animals to employ foreing 
bodies for protection, such as grains of sand or shells of 
other animals. 
