3io THE WONDER OF LIFE 



excreted. The animal is at the nadir of parasitic degener- 

 ation. But what of the life- history ? Out of the brood- 

 chamber there emerge Nauplius-larvse, with three pairs 

 of appendages, a food-canal, and a median eye. They feed 

 and grow and moult, and pass into a second the Cyprid 

 larval stage. These fix themselves, just like barnacles and 

 acorn-shells (see page 448), by means of their first pair of 

 feelers, to the back or limbs of young crabs, finding a soft 

 place at the base of the large bristles or setae. All but 

 the head region is cast off ; the structures within the head 

 contract ; eyes, tendons, pigment, and the remains of the 

 shell are all lost, and a tiny sac sinks into the interior of 

 the crab. Eventually it reaches the ventral surface of 

 the abdomen, and, as it approaches maturity, the cuticle 

 of the crab softens beneath it, so that the sac-like body 

 protrudes. It seems to live for three years, during which 

 the growth of the crab is arrested. The reproductive 

 organs of both male and female crabs are destroyed. 



Ox- Warbles. What an extraordinary story is that of 

 the ox- warble fly (Hypoderma bovis) I The eggs are laid 

 on the skin and are licked off into the mouth. According 

 to Jost, they hatch at the foot of the gullet, and the larvaa 

 bore into its wall and wander about in it for months (July- 

 November). They go on the march through the body, 

 through midriff, connective-tissue, kidneys, and what not 

 and come to rest beside the vertebrae (December-May). 

 Subsequently they pass upwards by way of the connective- 

 tissue of the back muscle to a position just below the skin 

 of the back the last chief place of their assembling. They 

 occur here from January till July, when they emerge and 

 fall on to the ground. They pass into the pupa stage on 

 the ground and the winged fly emerges in a few weeks. 



