Parasitism and Social Reciprocity. 187 



and the water flea, Cypris. Its second stage is called the pupa stage. 

 Here it is much like a corresponding stage in the Cypris, while the Bar- 

 nacle at that stage shows signs of divergence. In the pupa state it is 

 invested with a shell which is a bivalve with a hinge along the axis of 

 the back. In this it resembles both the C} 7 pris and the barnacle pupa. 

 Next the two front limbs become modified into feelers, or organs of 

 attachment, and the four hinder limbs are cast off and are succeeded by 

 six pairs of forked swimming feet. In these two particulars it imitates 

 the development of the Barnacle. But here they diverge. With the 

 help of his swimming feet and under the direction of his one eye, he 

 hunts a " host " and a square meal. This host is usually a hermit crab. 

 Into the intestines and among the liver tubes of this " host " the feelers 

 of the sacculina feel their way by means of numerous root-like ramifying 

 appendages, which are developed on them and through which the 

 digested nutriment is conveyed into the tissues of the sacculina. The 

 latter now having no further use for swimming organs nor eye nor sem- 

 blance of head, these are all aborted, and the parasite remains nothing 

 but a sac, shaped like a sausage or a discoidal bag, or perhaps like an 

 un symmetrical lump. It receives and expels water through a wide orifice 

 by which its tissues are oxidized, a process of quasi respiration ; but 

 beside this it has no other function than to mature the eggs for the next 

 generation. 



When this function is completed the sac drops off, leaving its root-like 

 feelers still in the host where they continue to live for some time totally 

 functionless, their occupation entirely gone. This animal never has a 

 stomach and never digests any food, but absorbing it from the tissues or 

 digestive organs of the host, by endosmose, it does nothing but assimi- 

 late it in its own tissues. Its youth is more noble than its old age, for 

 in youth it possesses at least two sense organs, sight and touch ; it has 

 the power of locomotion and some power of nervous co-ordination, a 

 will and choice. But it loses all these at maturity, and in old age it 

 has no more sense or sensibility than a turnip. 



The history of the Sacculina was observed to run parallel at first with 

 that of the Barnacle in many respects. The Barnacle, however, has at 

 first a mouth and stomach, and does some vigorous feeding in the Nau- 

 plius stage. Later, however, in the Pupa stage, this mouth is closed, 

 and it does not feed during the time occupied in finding a suitable place 

 of attachment, and in this stage it is very like the pupa of sacculina, 

 having, however, two eyes instead of one. But its subsequent life is 

 altogether different, for fixing itself to a log or ship, or some other inan- 

 imate object, it has to feed itself and digest its own food. So its mouth 

 is reopened, and stomach put in running order. Its fore legs, which 

 became feelers, as in the Sacculina, now become prehensile organs to 



