The larval development of the Cirratulidae is not well known, although an account of the development of Raphidrilus nemasoma Mont, was published by Sokolow in 1911 (under the name of Ctenodrilus branchiata), and of Audouinia tentaculata (Montagu) by Wilson in 1936. Apart from these two publications, only scattered observations may be found in the literature, and these are listed by Wilson (1936). Unfortunately, attempts to rear the larvae of Tharyx marioni (St Joseph) beyond the hatching stage have failed, and as it seems unlikely that further data may be gathered for some time, such results as have been obtained are presented here.

The main points of interest in the development and reproductive habits of T. marioni are: (i) the adults are atoquous; (ii) the eggs are comparatively large and yolky, and unlike Audouinia tentaculata, the pelagic stage is omitted; and (iii the larva on hatching is achaetous, as in other cirratulid larvae.