230 THE LIFE OF CRUSTACEA 



Copepoda taken by the tow-net in British seas, there 

 may sometimes be found species of the family 

 Monstrillidae (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-open- 

 ing ; 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 (Polychseta). The young are hatched as 

 nauplius larvae (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 the)' 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 blood- 

 vessels 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 



