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 (planul<z), 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 egg- 

 producing stage by E, the for- 

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



represent the succession of 

 stages, V-E being 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 



a, Two individuals from the fixed colonial stage (hydroid r ,1 , i 



zoophyte] each studded with knobbed tentacles, between Ol triC matter COttSlStS in til IS, 



which (in one individual) are buds which become liberated as fVaf anv O~lvPTl TTlPmHpr of" 

 medusas ; b, a medusa. / o 



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 

 (gonangium, 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 

 (gonopkores) which are never liberated. These buds may be 

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



