The scolex is biradially constructed, the proglottides flattened, quadrangular and bilaterally symmetrical.
The structure is bilaterally symmetrical, the body and shell elongated along the antero-posterior axis and nearly cylindrical.
The evolution of this cavity into a gut is foreshadowed in some Coelentera by the elliptical shape of the aperture, and by the development at its ends of a ciliated channel along which food is swept; we have only to suppose the approximation of the sides of the ellipse and their eventual fusion, to complete the transformation of the radially symmetrical Coelenterate into a bilaterally symmetrical Coelomate with mouth and anus at opposite ends of the long axis.
The original form was bilaterally symmetrical, and this symmetry is retained in all the classes except the Gastropoda.
In all other Siphonophores, the ancestral form was a Siphonula, a bilaterally symmetrical Anthomedusa After Haeckel, from Lankester's Treatise on Zoology.
The shapes and colours are extremely varied; bilaterally symmetrical forms are most frequent with red, blue or violet colours.
Their most important characteristic in comparison with the latter is that they are, both in external and internal structure, bilaterally symmetrical.
The prothallus and sexual organs may resemble those of the Polypodiaceae; in Aneimia and Mohria the prothallus, though flattened, is not bilaterally symmetrical, the growing point being on one side; a filamentous type of prothallus is known in Schizaea.
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.
In association with this movement in a definite direction the body has become vermiform and bilaterally symmetrical.