Manchester Memoirs, Vol. Ix. (1916). 3 



of sedentary habit, such as the Cirripedes, Tunicata and 

 Polyzoa, that show a predominant symmetry of the same 

 kind. This symmetry can only be accounted for by appeal 

 to their phylogenetic history. It can be shown in all cases 

 that these sedentary animals with a bilateral symmetry 

 are derived from ancestral forms that had a previous 

 history of free movement in a definite direction. But 

 even in these cases there is a tendency for certain organs, 

 and particularly those that are connected with the inges- 

 tion of food, to assume a secondary radial symmetry. 



The six pairs of legs of the Cirripede when in use are 

 not arranged in a bilateral series but form an almost 

 regular funnel towards the apex of which the food is 

 passed and the shells frequently assume an almost perfect 

 radial symmetry. Similarly the lophophore of Polyzoa 

 and the tentacles round the mouth of Tunicates usually 

 assume a radial symmetry when in action. 



Jn the Coelenterate we find that the plan of the body 

 is usually that of a complete radial symmetry. This 

 radial symmetry is expressed by the arrangement of the 

 organs along a certain number of rays proceeding from a 

 common centre. The body may be cylindrical, spherical, 

 or disc-shaped or any intermediate kind of solid radially 

 symmetrical form, but there are no ends that we can call 

 anterior or posterior, no right or left sides, and no dorsal 

 or ventral surfaces. We can usually distinguish an oral 

 end or an oral surface where the mouth is situated from an 

 aboral end or aboral surface opposed to it, but it is an 

 inaccurate use of terms to speak of either of these sur- 

 faces as anterior or dorsal as is sometimes done. 



The radially symmetrical form is always correlated 

 with one of two habits of life, the sedentary habit or the 

 floating habit. As examples of animals of the former 

 habit we may refer to such examples as Hydra, the Sea- 



