- " Errant" Polychaetes with well-marked prostomium possessing tentacles and palps with evident and locomotor parapodia, supported (with few exceptions) by strong spines, the aciculi; muscular pharynx usually armed with jaws; septa and nephridia regularly metameric and similar throughout body; free living and predaceous.
It is important to notice that the metameric plan of growth of Chaetopods is still preserved.
According to Heymons, the yolk-cells must be Metameric Segmentation.
In one family (Ligulidae) the segmentation is only expressed in the metameric distribution of the generative organs and the worm is externally unisegmental.
In the latter respect, and in the fact that they frequently develop by a metamorphosis, they approach the Mollusca, but they differ from that group notably in the occurrence of metameric segmentation affecting many of the systems of organs.
Palaeontology therefore throws no light on the question whether the metameric or the unsegmented Mollusca were the earlier.
On the other hand, there is no evidence that the forms which show no metamerism, such as the Gastropoda, are descended from metameric ancestors.
The metameric repetition of the shellplates and of the ctenidia are probably special modifications, but it is difficult to explain the spicules of the dorsal integument except as a condition more primitive than the shell itself.
The Cephalopoda can be derived without much difficulty from the schematic Mollusc, if we assume that some metameric repetition of organs has occurred, as explained above in reference to the coelom.
In the true Chitonidae there are generally two apertures on each side, and in two species three or four, another instance of the tendency to metameric repetition in the group. The auricles are connected with one another posteriorly behind the ventricle.
On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side.
And similarly in the development of a complicated organism, the suppression or doubling of a single cell or group of cells may bring about striking differences in the symmetry of the adult, or the reduction or increase in the number of metameric organs.
Much of the character of organisms is due to various symmetries, radial, bilateral, metameric and so forth, and these symmetries arise, partly at least, from the mode of growth by cell division and the marshalling of groups of cells to the places where they are destined to proliferate.
To the right, strobila condition of the scyphistoma, consisting of thirteen metameric segments; the uppermost still possesses the sixteen tentacles of the scyphistoma; the remainder have no tentacles, but are ephyrae, each with eight bifid arms (processes of the disc).
2, 9), with its metameric transverse grooves and flagella.