C S 5 s 3 
XVI. On the structure of the organs of respiration in animals 
which appear to hold an intermediate place between those of the 
class pisces and the class vermes, and in two genera of the last 
mentioned class . By Sir Everard Home, Bart. V. P. R. S. 
Read June i, 1815. 
From all the facts in comparative anatomy with which we 
are acquainted, there is reason to believe, that the great 
scheme of the animal creation is composed of one uniform 
gradation of structures, and it is only by collecting the diffe- 
rent appearances met with in the same organs of different 
animals, into regular series, that a solid basis can be laid on 
which a general system may be constructed. 
In this view of the subject, every link that is added to any 
one series, acquires a value ; as it increases, in however small 
a degree, the foundation upon which an edifice of such im- 
portance is to be raised, and therefore may not be undeser- 
ving of the attention of the Society. 
In fishes, the mode of respiration by means of gills is well 
understood, and there is probably no better criterion by which 
an animal may be allowed to belong to that class, than its 
having gills. I11 the class vermes, confining our observa- 
tions to those genera that live under water, the respiratory 
organs commonly met with, are of two kinds, one internal 
as in the genus Teredo, the other external as in the genus 
Amphitrite, both of which I have described to the Society 
upon a former occasion.* 
* See PhU. Trans. Vol. Ixxv. p. 333. 
