116 THE NATURE AND VARIETIES OF 



Distomata, derived from them in turn. The former never 

 exhibit clearer indications of animality than a power of 

 peristaltic motion, and some rudimentary organs, external 

 and internal ; and they range from this down to simple rigid 

 tubes mere bud-sacs, quite destitute of organization and 

 all power of motion. The latter, again, possess as complex 

 a structure as is found anywhere in the class of animals to 

 which they belong and this not as an occasional develop- 

 ment, but uniformly in all cases, and quite irrespective of 

 the organization of the precursory form from which they 

 have been derived. The derivative zooid, indeed, is not 

 only much more highly organized than the primary, but it 

 is truly the typical form of the species, as appears by its 

 correspondence with the fully developed condition of those 

 Trematoda in which " alternation" does not prevail, and 

 consequently but one form occurs in the same species. 



2. The second point is that not only are the precursory 

 zooids destitute of all sexual characters, but that no such 

 organs are developed even in the derivative forms, till some- 

 time after they have acquired the typical conformation in 

 all other particulars. The appearance of these organs, in 

 fact, constitutes a new epoch in the life-history of the 

 typical form, quite as observable as its derivation from the 

 preceding one, and admits of our dividing the whole succes- 

 sion of changes into the three stages to which the terms 

 Protomorphic, Orthomorphic, and Gamomorphic, were ap- 

 plied in the Introductory Chapter. 



From the consideration of these points, we may assign the 

 following as the characters positive and negative of that 

 modification of " alternation," which prevails among the 

 Trematoda : 



1. That the " alternation" consists in the derivation of 

 sexual and oviparous zooids, from the last generation of a 

 series of others which are sexless and gemmiparous. 



2. That the later set of zooids, which eventually acquire 



