A copepod which Brady named Fucitrogus rhodymieniae and which burrows into the fronds of the seaweed Rhodymenia palmata devouring the tissues and producing gall-like swellings is shown to be the naupliar stage of the harpacticid genus Thalestris.
The nauplius is highly specialized, having appendages unlike those of nauplii of other copepods and admirably suited to its life in the alga. The second antenna is masticatory in function and has a powerfully developed mandible like gnathobase.
The skin of the nauplii is soft and extensible so that the animal as a whole appears to grow continuously; but there are six naupliar stages, and they show a doubling of volume of the hard parts at each instar with remarkably little increase in the complexity of the appendages.
The first copepodid is advanced, as far as the number of the appendages is concerned; but these are under-developed in structure and apparently not functional, the animal at this stage living on internal stores laid in by the nauplius.
The last (apparently second) copepodid has normal appendages, is the first stage normally to be found in an open gall and is probably able to feed.
The adult male and female are described and the species is compared with T. purpurea Sars which is closely related.