Coelom generally reduced to a system of tubes, sometimes communicating with vascular system; in Acanthobdella and Ozobranchus a series of metamerically arranged chambers as in Oligochaeta.
Furthermore, in the mid region of the body this coelom is broken up by metamerically arranged septa, as in Acanthobdella.
This circlet of gill-lamellae led Cuvier to class the limpets as Cyclobranchiata, and, by erroneous identification of them with the series of metamerically repeated ctenidia of Chiton, to associate the latter mollusc with the former.
The body is not metamerically segmented and is composed of a muscular tunic covered externally by a more or less modified cellular layer.
The setae are implanted metamerically in accordance with the metamerism of the body, which consists of a prostomium followed by a number of segments.
They are metamerically placed, and belong to the same metamere as the digestive caeca, thus alternating with the generative sacs.
The gills are thus metamerically repeated; there may be from four to eighty pairs, but there is these, and all round the animal, is the mantle-skirt.
In the Metanemertini, where the longitudinal stems lie inside the muscular body-wall, definite and metamerically placed nerve branches spring from them and divide dichotomously in the different tissues they innervate.