2(5 MANUAL OF ZOOLOGY. 



ovum gives rise to a primitive polypide which repeats itself by 

 a process of continuous gemmation, until an entire colony is 

 produced, each member of which is independent of its fellows, 

 and is capable of producing ova. In such a case, therefore, 

 the term u individual " must be applied to the entire colony, 

 since this is the result of the development of a single ovum. 

 The separate beings which compose the colony are technically 

 called " zooids." In like manner, the Hydra which produces 

 fresh and independent Hydrse by discontinuous gemmation, is 

 not an " individual," but is a zooid. Here the zooids are not 

 permanently united to one another, and the " individual" Hydra 

 consists really of the primitive Hydra, plus all the detached 

 Hydrae to which it gave rise. In this case, therefore, the " indi- 

 vidual " is composed of a number of disconnected and wholly 

 independent beings, all of which are the result of the develop- 

 ment of a single ovum. It is to be remembered that both the 

 parent zooid and the "produced zooids" are capable of giving 

 rise to fresh Hydras by a true generative process. It must 

 also be borne in mind that this production of fresh zooids by a 

 process of gemmation is not so essentially different to the true 

 sexual process of reproduction as might at first sight appear, 

 since the ovum itself may be regarded merely as a highly spe- 

 cialised bud. In the Hydra, in fact, where the ovum is pro- 

 duced as an external process of the wall of the body, this like- 

 ness is extremely striking. The ovarian bud, however, differs 

 from the true gemmae or buds in its inability to develop itself 

 into an independent organism, unless previously brought into 

 contact with another special generative element. The only 

 exceptions to this statement are in the rare cases of true "par- 

 thenogenesis," to be subsequently alluded to. 



b. Reproduction by Internal Gemmation. Before considering 

 the phenomena of " alternate generations," it will be as well to 

 glance for a moment at a peculiar form of gemmation exhi- 

 bited by some of the Polyzoa, which is in some respects inter- 

 mediate between ordinary discontinuous gemmation and 

 alternation of generations. These organisms are nearly allied 

 to the sea-mat, already spoken of, and, like it, can reproduce 

 themselves by continuous gemmation (forming colonies), by a 

 true sexual process, and rarely by fission. In addition to all 

 these methods they can reproduce themselves by the formation 

 of peculiar internal buds, which are called " statoblasts." These 

 buds are developed upon a peculiar cord, which crosses the 

 body-cavity, and is attached at one end to the fundus of the 

 stomach. When mature they drop off from this cord, and lie 

 loose in the cavity of the body, whence they are liberated on 



