628 THE POPULAR SCIENCE MONTHLY. 



The young Hydra, like its mother, is a simple sac with its wall com- 

 posed of a double layer of cells, the cavity or stomach communicating 

 directly with the stomach of the mother, so that the contractions of the 

 body carry all the food taken by one into the stomach of the other, 

 and inversely (Fig. 1). The parent and child live awhile in this way, 

 but, whenever the latter has reached a certain size, it is detached and 

 fixes itself on some near object, where it hunts on its own account. 

 Soon the parent and offspring are indistinguishable, and during the 

 summer they never cease to produce new Hydras. But, sometimes, in 

 fertile waters rich in game, each Hydra retains its progeny, the little 

 ones grow and produce new Hydras in their turn, and thus a new 

 colony is founded. Trembly kept a long time a Hydra that carried 

 twenty-two young ones of four different generations — a living genea- 

 logical tree. 



That which is accidental in the common Hydra is quite normal 

 with another fresh-water species, the Cordylophora lacustris, and in 

 most marine Hydroids, in which the colonies often consist of innumer- 

 able individuals. But then new phenomena are seen. The social life 

 becomes complicated, and a true division of labor occurs among the 

 members of the same colony. At first all were alike, performing the 

 same functions in the same manner. Specialization soon begins : some 

 hunt, others digest, others reproduce ; so that individuals that at first 

 had no need of each other and lived united only in a careless way, be- 

 come reciprocally necessary ; the society thus acquires coherence and 

 solidarity. In the Hydractinia we count not less than seven sorts of 

 individuals : 1. Nourishers or gasterozoids ; 2. Prehensers or dacty- 

 lozoids, provided with bunches of stinging capsules ; 3, Dactylozoids 

 without stinging capsules ; 4. Defenders ; 5. Reproducers of individ- 

 uals of both sexes ; 6. Males ; 7. Females. They are different in 

 shape as well as in function ; each taking the figure suited to its work, 

 rising or falling in organization ; so that division of labor brings with 

 it, as in human society, inequality of conditions. The species thus be- 

 come polymorphic. 



Of these seven sorts of individuals that compose a colony of Hy- 

 dractinia, the nourishers alone seem capable of living by themselves. 

 The others have neither mouth nor tentacles, the sexual individuals 

 are reduced to simple sacs, the defenders seem to be only sharp spines, 

 between which the polyps can hide themselves (Fig. 5). It may seem 

 an exaggeration to attribute individuality to these different parts. It 

 may be said that they are simply organs ; but organs of what ? They 

 are just as independent of each other, just as independent of the nourish- 

 ers, as the latter can be of one another. They are, then, not organs of 

 those Polyps. Can they be organs of the colony ? It is already under- 

 stood that the colony has the character of an individual, and the trans- 

 formation we seek to demonstrate is admitted. But how can a colony 



