50 INTRODUCTION. 



were separated as a distinct order by these observers, on account 

 of their possession of a second orifice to the alimentary canal, 

 and the general tendency of their plan of organization to that 

 which characterizes the inferior Mollusca. The importance of 

 this distinction was at once recognized ; and the group received 

 the designation of Polyzoa from Mr. J. Y. Thompson, and of 

 Bryozoa from Prof. Ehrenberg. The organization of this very 

 interesting group was further elucidated, some years v subsequent- 

 ly, by the admirable observations of Dr. Arthur Farre upon a 

 newly-discovered form (named by him Boiverbankia), the trans- 

 parence of whose envelopes allowed its internal structure to be 

 distinctly made out ; and the additional features which he de- 

 tected, were all such as to strengthen the idea already entertained 

 of its essentially Molluscan character. This idea received its final 

 and complete confirmation from the admirable researches of 

 M. Milne Edwards on the Compound Ascidians, which are the 

 lowest animals whose Molluscous nature had been previously ac- 

 knowledged ; these having been discovered by him to agree with 

 Zoophytes in their plant-like attribute of extension by " gemma- 

 tion" or budding, and to present, in all the most important fea- 

 tures of their organization, an extremely close approximation to 

 the Bryozoa. Thus whilst Microscopic research has degraded 

 the Foraminifera from their supposed rank with the Nautilus 

 and Cuttle-fish, to the level of the Sponge, it has raised the 

 "Wheel-Animalcules into proximity with the aquatic "Worms, 

 and the humble " Sea-Mat," formerly supposed to be a Plant, to 

 a position not much below that of the Oyster and Mussel. 1 



Another most curious and most important field of microscopic 

 inquiry has been opened up in the discovery of the Transforma- 

 tions which a large proportion of the lower animals undergo, 

 during the early stages of their existence ; and notwithstanding 

 that it has even yet been very imperfectly cultivated, the unexpect- 

 ed result has been already attained, that the fact of " metamor- 

 phosis," previously known only in the cases of Insects and Tad- 

 poles, and commonly considered as an altogether exceptional phe- 

 nomenon, is almost universal among the inferior tribes ; it being 

 a* rare occurrence for the offspring to come forth from the egg in 

 a condition bearing any resemblance to that which characterizes 

 the adult, and the latter being in general attained only after a 

 long series of changes, in the course of which many curious 

 phases are presented. One of the earliest and most remarkable 

 discoveries which was made in this direction, that of the meta- 

 morphosis of the Cirrhipeds (Barnacles and their allies) by Mr. J. 

 Y. Thompson, proved of most important assistance in the de- 

 termination of the true place of that group, which had previously 

 been a matter of controversy ; for although in their outward 



dead horny polypidoms which you hold may be, the two species of animal which have 

 formed them, are at least as far apart in the scale of creation as a quadruped is from a 

 fish." 



1 With reference to the subjects of the three preceding paragraphs, see Chaps. IX, 

 X, XI. 



