^GROWTH AND ASEXUAL GENESIS. 4'2a 



general truth with a difference. In the smaller members the 

 agamogenesis is incomplete, and iu the larger it disappears. 

 Each sub-section of the Molliiscoida shows us this. The gemma- 

 tion of the minute Polijzoa, though it does not end in the sepa- 

 ration of the young individuals, habitually goes to the extent 

 of producing families of partially-independent individuals ; 

 but their near allies the Bi achiopoda, which immensely exceed 

 *;hem in size, are solitary and not gemmiparous. So, too, is 

 it with the Ascidiokla. And then among the true MoUusca, 

 including all the largest forms belonging to this sub-IdngJom, 

 no such thing is known as fission or gemmation. 



Take next the Annulosa, including under this title the 

 Annuloida. When treating of morphological composition, 

 reasons were given for the belief that the annulose animal is 

 an aggregate of the third order, the segments of which, 

 produced one from another by gemmation, originally 

 became separate, as they still become in the cestoid 

 Entozoa ; but that by progressive integration, or arrested 

 disintegration, there resulted a type in which many such 

 segments were permanently united (§§ 205-7). Part of the 

 evidence there assigned, is evidence to be here repeated in 

 illustration of the direct antagonism of Growth and Asexual- 

 Genesis. We saw how, among the lower Annelids, the string 

 of segments produced by gemmation presently divides trans- 

 versely into two strings ; and how, in some cases, this resolu- 

 tion of the elongating string of segments into groups that 

 are to form separate individuals, goes on so activel}' that as 

 many as six groups are found in different stages of progress 

 to ultimate independence — a fact implying a high rate of 

 fissiparous multiplication. Then we saw that, in the superior 

 annulose types, distipguished in the mass by including the 

 larger species, fission does not occur. The higher Annelids 

 do not propagate in this way ; there is no known case of new 

 individuals being so formed among the M>iriapoda ; nor do 

 the Crustaceans aflford us a single instance of this primordial 

 mode of increase. It is, indeed, true that while 



