232 ANIMAL PEDIGREES 



loosely fitting outer skin, with a single orifice 

 through which the young escape. A nervous 

 system is present, but there are no traces of limbs, 

 digestive system, heart, breathing organs, or sense 

 organs. Indeed, examination however careful of 

 an adult Sacculina would fail to afford any clue as 

 to its real zoological affinities. 



Development however in obedience to the potent 

 law of recapitulation, shows us at once that 

 Sacculina is a Crustacean, more closely allied to the 

 Barnacles than to any other of the more familiar 

 members of the group. From each of the exceed- 

 ingly numerous eggs which a Sacculina produces, 

 there emerges a minute, free swimming larva of the 

 type known as a Nauplius, characterised by possess- 

 ing a somewhat pyriform body, a single median eye, 

 and three pairs of swimming appendages or legs. 

 Nauplius larvae are widely spread amongst Crusta- 

 ceans. All Entomostraca, except the Cladocera, 

 hatch in this form, and Nauplius larvae are found 

 in individual members in nearly all the higher 

 groups as well. The only special peculiarity about 

 the Sacculina Nauplius is that it has neither mouth 

 nor digestive organs of any kind, these being 

 unnecessary by the presence of a considerable 

 quantity of food yolk. The Sacculina larva con- 

 tinues its free existence for a time; it casts its skin, 

 or rather its cuticle many times, emerging each time 

 rather more complicated in structure though actually 

 smaller in size, for it cannot yet take in food. 

 Ultimately it reaches the condition spoken of as the 

 pupa stage. The pupa is enclosed in a bivalved 



