35° 



ANIMAL LIFE-HISTORIES 



vegetative means, and the other by eggs. In this case the former 

 is the fixed branching colony {hydroid zoophyte), which not only 

 buds to increase its size, but also produces special buds which are 

 liberated as jelly-fishes or medusae. These produce eggs, which 

 develop into oval ciliated free-swimming larvae [planula;), some- 

 thing like those already described for Sponges (see p. 342). After 

 a time each larva fixes itself to some firm body, and becomes an 

 individual essentially resembling a Freshwater Polype {Hydra). 



By means of budding, a 

 fixed colony (hydroid zoo- 

 phyte) arises from this; later 

 on, special buds grow into 

 medusae, and so on. If the 

 hydroid zoophyte or vegeta- 

 tive stage be represented by 

 v, and the medusa or es'S'- 

 producing stage by E, the for- 

 mula V-E-V-E-V-E, &c. will 

 represent the succession of 

 stao'es, v-E beinsf a single 

 cycle or life-history. But we 

 cannot say that this is the 

 life -history of an individual, 

 since both v and e are indi- 

 viduals. And the singularity 

 of the matter consists in this, 

 that any given member of 

 the series closely resembles 

 its grandparents and grandchildren, but differs greatly from its 

 parents and children. 



In some of the Hydroid Zoophytes there are special arrange- 

 ments for protecting the medusa-buds while they are being de- 

 veloped. Instead of projecting freely into the surrounding sea- 

 water, a group of them is sheltered in a little horny case 

 {^gonangitnn, fig". 868). 



It must not be supposed that all of the numerous species of 

 Hydroid Zoophytes give rise to free-swimming jelly-fishes, for 

 the egg -cells and sperms are often produced in special buds 

 [gonophores) which are never liberated. These buds may be 

 precisely like jelly-fishes in structure, obscurely like, or quite 



Fig. 867.— Life-history of Syncon'ne 



a. Two individuals from the fixed colonial stage \hydroid 

 zoophyte) each studded with knobbed tentacles, between 

 which (in one individual) are buds which become liberated as 

 medusa;; b, a medusa. 



