CCELENTERATA 143 



tached, usually by the formation of a supporting skele- 

 ton. In many hydroids a hard wall is formed on the 

 outside of the animal, but in corals it is internal as 

 well (Fig. 80). These animals can not move from 

 place to place, but they can contract the soft part of the 

 body. Corals are like sea anemones in structure except 

 for the skeleton formed in corals by the deposit of 

 calcareous salts in the mesoderm. 



Nervous System. — Most of the fixed forms have no 

 nervous system ; the surface of the body is, however, ex- 

 tremely sensitive to contact. The motile forms have a 

 definite nervous system composed of a central nerve 

 ring from which branches are given off. Some of 

 them have also simple sense organs which are sensitive 

 to light and sound vibrations. 



Colonial Forms. — Many forms develop buds which 

 do not become detached ; these grow into distinct in- 

 dividuals, which live together in a colony. Frequently 

 members of a group formed in this way become adapted 

 to the performance of special work, and the colony 

 comes to resemble a more highly-differentiated, many- 

 celled animal. It becomes a question then whether the 

 whole colony is to be regarded as a collection of sepa- 

 rate individuals which live together for the common 

 good, or as a single animal with exceedingly well-de- 

 veloped parts (Fig. 81). 



Reproduction. — In all of these forms reproduction 

 is either non-sexual (by budding, a process allied to 

 fission) or sexual (by the union of dissimilar cells). 

 In budding a mass of cells grows out with the power to 

 develop into a new individual which may, or may not, 



