Functional design and evolution of the polychaete Aphrodite aculeata

The body form of Aphrodite is attributed to its exploitation of the nereid creeping mechanism. It employs a fast stepping pattern, individual limb movements being extension and retraction of the neuropodium with coordinated chaetal movement. The downward and backward propulsive stroke raises the worm clear of a hard substratum. The notopodium is stabilized and its long chaetae raised and lowered in defence. The segments are short but wide. The body wall muscles, relieved of locomotory function, are a basketwork of longitudinal and diagonal muscles capable of maintaining constant body dimensions. Muscles of the appendages originate from stabilized points on the body wall. Trunk septa are reduced, in association with the constant body shape, but the first few septa form a membrane enclosing the proboscis. Complete and muscular septa occur in the rectal segments. During respiratory movements the elytra are depressed by intrinsic and extrinsic elytrophore muscles and elevated by coelomic pressure. Coelomic pressure drops immediately before elytral depression and is controlled by the body wall muscles, especially the diagonal muscles. Aphrodite differs from its near relative Hermione in the reduction of septa and simplification of body musculature.