In all the cases mentioned, the animal is parasitic in the final state of its existence--at least in the female sex--but there are a few Copepoda which are free-swimming, both when young and when adult, but parasitic in the intermediate stages. Among the Copepoda taken by the tow-net in British seas, there may sometimes be found species of the family Monstrillidæ (Fig. 74, F), which are remarkable for having no appendages between the antennules and the first pair of swimming feet. They have no trace of jaws, and only a minute vestige of a mouth-opening; while internally there is no food-canal, so that the animals are incapable of taking nourishment. Their development was for long a mystery, but it is now known that the greater part of their life is passed as internal parasites in certain bristle-footed worms (Polychæta). The young are hatched as nauplius larvæ (Fig. 74, A) without mouth or food-canal, but capable of swimming, and having the third pair of appendages (mandibles) furnished with strong hooks, by means of which they fasten on to the worm which is to serve as their host. The nauplius bores through the skin of the worm, casting its cuticle and losing all its appendages in the process, and making its way into one of the bloodvessels in the form of a little oval mass of cells (Fig. 74, B), within which no organs except the degenerating nauplius eye can be detected. It later becomes enclosed in a delicate cuticle, and from one end two long finger-like processes grow out, which are believed to have the function of absorbing nourishment from the blood of the host (Fig. 74, C, D). Within the cuticle the organs of the adult animal are gradually differentiated (Fig. 74, E), and when fully formed it bores its way through the tissues of its host by means of rows of hook-like spines surrounding the pointed posterior end of the sac. On reaching the surface the enclosing membrane bursts, and the adult animal is set free.