ALTERNATION OF GENERATIONS. 119 



many successive crops of gemmae* ; whereas the rudi- 

 mentary forms of the Trematoda seem to pass away as soon 

 as they have thrown off all their freight of gemmae, whose 

 production is the great object of their existence. The con- 

 trast is equally marked in the derivative zooids, though in 

 the opposite way. In the Trematoda, where they are re- 

 presented by the distoma-form, it is these that have the 

 greater length of life ; for a considerable time generally 

 elapses before they go through their metamorphosis and 

 acquire sexual organs while the medusoids of the zoo- 

 phytes, which are developed for the very purpose of repro- 

 duction, have a correspondingly brief duration, their term 

 of life coming to a close as soon as the spermatic and ger- 

 minal products are matured and discharged. 



There is besides a fragmentary and subsidiary character 

 attaching to these medusoids in many cases, which of itself 

 suggests the idea of their being merely detached organs for 

 the formation and fecundation of ova. It is true that there 

 are great differences among them in this respect greater 

 even than among the precursory zooids of the Trematoda. 

 The variability is such that while some of the medusoids 

 have an organization apparently much more complex than 

 that of the polype-stock, in other cases the sexual zooids are 

 reduced to mere sperm and germ-sacs, of the most rudi- 

 mentary description, without any trace of medusoid struc- 

 ture, though the correspondence of the two extremes is 

 established by a continuous series of intermediate forms. 



There is an equal variability in the isolation or attachment 

 of the sexual structures. In general these are thrown off 

 as free zooids, but in many species they are never detached, 

 forming fixed appendages of the parent stock, and gradually 

 merging into the character of mere organs of reproduction. 

 So completely is this the case in the common Hydra, that 



* Carpenter's Compar. Physiology, 4th Ed., p. 559. 



