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 silica is employed 

 in the formation of a skeletal system, but this substance is 

 confined to the Protozoa and Cmlenterata, 



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 keratin 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. 



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 integumentary 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 forcing 

 bodies for protection, such as grains of sand or shells of 

 other animals. 



