PARASITES AND MESSMATES 233 



and Mr. Geoffrey Smith. The nauplius larva 

 (Fig. 75, A) resembles that of the normal Cirripedes, 

 especially in the shape of the dorsal shield, which 

 is drawn out on either side in front into a pair of 

 fronto-lateral horns. It has, however, no mouth, 

 and the food-canal is quite absent. As in the normal 

 Cirripedes, the nauplius is followed by a cypris stage 

 (Fig. 75, B), also mouthless, and it is in this form 

 that the Sacculina seeks the Crab on which it is to 

 become parasitic. It would be almost impossible for 

 the cypris larva to settle on that part of the Crab 

 where the adult Sacculina is afterwards to appear, 

 since the Crab usually has its abdomen closely 

 pressed against the under-side of its thorax. The 

 larva therefore attaches itself on some exposed part 

 of the Crab, often on one of the legs, clinging to a 

 hair by means of its antennules. It bores through 

 the cuticle at the base of the hair, and the contents 

 of its body pass into the interior of the Crab as a 

 little mass of cells, the empty cypris shell being cast 

 off. This mass of cells, which constitutes the embryo 

 Sacculina, is carried about by the blood-currents of 

 the Crab till it reaches the under-side of the intestine, 

 where it becomes attached. It now begins to send 

 " out roots (Fig. 76), and as it grows the central mass 

 travels backwards along the intestine of the Crab till 

 it reaches the place where the adult parasite is to 

 emerge. As the mass increases in size, and the 

 organs of the Sacculina become differentiated within 



