SYMMETRY. 23 
tend to a cylindrical form. In this type we can distinguish 
axtal and peripheral parts and the two ends of the main axis 
can usually be recognised as the afex and the Jase. The 
mouth is situated at the one end or apex often termed the 
orad end, the base being known as the adoraZend. Examples 
are found among the lowest animals (Protozoa, Porifera and 
Calenterata) 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 vight and “eft side. The 
parts are either median or lateral. Nearly all the members 
of the animal kingdom above the Cwlenterata conform 
more or less closely to this type. 
Certain organisms do not appear to conform to any of 
these types. Ameba and some other low organisms have 
no definite shape of body since they change their shape at 
everymoment. 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 plano-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 ce//. 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. 
