SYMMETRY. 23 



tend to a cylindrical form. In this type we can distinguish 

 axial and peripheral parts and the two ends of the main axis 

 can usually be recognised as the apex and the base. The 

 mouth is situated at the one end or apex often termed the 

 oral end, the base being known as the aboral end. Examples 

 are found among the lowest animals {Protozoa^ Porifera and 

 Ccelenterata) which are either sedentary (fixed by the aboral 

 end) or pelagic. 



3. Plano-symmetric (bilaterally symmetric) animals 

 have their parts arranged about a central plane^ which 

 usually lies in the long axis of the body. In these we 

 can determine an anterior and a posterior end, a dorsal 

 and a ventral surface, and a right and left side. The 

 parts are either median or lateral. Nearly all the members 

 of the animal kingdom above the Ccelenterata conform 

 more or less closely to this type. 



Certain organisms do not appear to conform to any of 

 these types. Amoeba and some other low organisms have 

 no definite shape of body since they change their shape at 

 every moment. They really belong to the centro-symmetric, 

 because, when encysted or subjected to a stimulus, they 

 assume the spherical shape. Other higher types, such as 

 the snail, show a distortion which destroys to some extent 

 the piano-symmetry underlying their general body-form. 



The locomotion of animals usually conforms to their 

 symmetry. Most centro-symmetric animals rotate freely 

 about the centre, but do not move in a definite direction. 

 Axo-symmetric animals, if not sedentary, usually move in 

 the direction of the axis of symmetry, and plano-symmetric 

 animals usually move in the direction of the plane of 

 symmetry, with the anterior end forwards and usually with 

 the plane of symmetry vertical. 



Morphological Units. — If we next proceed to in- 

 vestigate the actual constituent elements of organisms we 

 can discern a very definite unit which occurs throughout. 

 This unit is called a cell. It is impossible to define a cell in 

 such a way as to include all the numerous forms and 

 modifications, but for our present purpose we must regard 

 it as a definite mass of protoplasm containing a nucleus, 

 and usually having more or less of a limiting cell-membrane. 



