Chaetal arrangement and homology in spionids (Polychaeta : Spionidae)

In anterior segments of spionid polychaetes notochaetae are generally capil laries arranged in three distinct groups or rows including an anterior and a posterior row and a dorsal superior tuft; neurochaetae are also capillari es arranged in anterior and posterior rows plus a ventral inferior tuft. In succeeding segments, the notopodial capillaries are reduced in number with the groups becoming indistinct, the neuropodial capillaries are usually re placed by hooks in differing patterns. These patterns of replacement includ e those with replacement only in the posterior row, in both anterior and po sterior rows, or of all neuropodial capillaries. A pattern of alternating n europodial capillaries alternating with the neuropodial hooks is described for the first time. In most polydorid taxa the chaetal arrangement on segme nt 5 is changed from a vertical to a diagonal or horizontal position, presu mably associated with the elongation of the segment. The modified chaetae a re usually arranged in two curved diagonal or horizontal rows corresponding to the anterior and posterior rows of notochaetae in other spionids. Based on positional criteria different kinds of spionid chaetae can be shown to be homologous making it possible to code them consistently for a cladistic analysis.