444 LAWS OF MULTIPLICATION. 



this gemmiparous multiplication is from time to time inter- 

 rupted by a transverse splitting-up of the body into segments, 

 which successively separate and swim away: the result of 

 the two processes being that, in the course of a season, there 

 are produced from a single germ great numbers of young 

 Medusce, which are the adult or sexual forms of the species. 

 Eespecting ccelenterate animals of this degree of composi- 

 tion, it may be added that when we ascend to the larger 

 kinds we find asexual genesis far less active. Though com- 

 parisons are interfered with by differences of structure and 

 mode of life, yet the contrasts are too striking to have their 

 meanings much obscured. If, for instance, we take a solitary 

 Actinozoon and a solitary Hydrozoon, we see that the rela- 

 tively-great bulk of the first, goes along with a relatively- 

 slow agamogenesis. The common Sea-anemones are but 

 occasionally observed to undergo self-division : multiplication 

 by budding being in some cases largely followed, but their 

 numbers are not rapidly increased by either process. A 

 higher class of secondary aggregates exemplifies the same 

 general truth with a difference. In the smaller members the 

 agamogenesis is incomplete, and in the larger it disappears. 

 The gemmation of the minute Polyzoa, though it does not 

 end in the separation of the young individuals, habitually 

 goes to the extent of producing families of partially-inde- 

 pendent individuals ; but their near ally, the Phoronis, which 

 immensely exceeds them in size, is solitary and not gemmi- 

 parous. So, too, is it with the Ascidioida. And then among 

 the true Mollusca, which are relatively large, no such thing 

 is known as fission or gemmation. 



Take next the Annulosa, including under this title the 

 Annelida and Arthropoda. When treating of morphological 

 composition, reasons were given for the belief that the annu- 

 lose animal is an aggregate of the third order, the segments 

 of which, produced one from another by gemmation, ori- 

 ginally became separate; but by progressive integration, or 

 arrested disintegration, there resulted a type in which many 



