Morphology and fine structure of two marine tubificids (Oligochaeta), closely related to the gutlessPhallodrilus spp.

Abstract

The morphology and ultrastructure of the two interstitial marine tubificidsPhallodrilus prostatus andAktedrilus monospermathecus (Oligochaeta), closely related to the gutlessPhallodrilus spp., is described. Since pertinent histological descriptions of ‘regular’ marine tubificids were found to be non-existent, this study provides a structural basis allowing comparisons both with limnetic species and the recently described marine anenteric forms. Compared to the few freshwater tubificids investigated histologically, major differences are the substitution of the dorsal blood vessel by a large sinus, the close contact between nephridial tissue and the subintestinal vessel, a characteristic ‘wavy’ arrangement of cuticular collagen fibres, and the formation of branched epicuticular projections. The structural divergences from the gutless relatives refer mainly to the body wall, the vascular system and its endothelial structure. The various peculiarities outlined here indicate that generalizations of the situation as found in the few histological studies are as yet problematical.