In association with this movement in a definite direction the body has become vermiform and bilaterally symmetrical.
In all other Siphonophores, the ancestral form was a Siphonula, a bilaterally symmetrical Anthomedusa After Haeckel, from Lankester's Treatise on Zoology.
The Lamellibranchia are mainly characterized by the rudimentary condition of the head, and the retention of the primitive bilateral symmetry, the latter feature being accentuated by the lateral compression of the body and the development of the shell as two bilaterally symmetrical plates or valves covering each one side of the animal.
The original form was bilaterally symmetrical, and this symmetry is retained in all the classes except the Gastropoda.
Their most important characteristic in comparison with the latter is that they are, both in external and internal structure, bilaterally symmetrical.