THE ZOONITES, 279 
in the same symmetrical way. A main canal passes down the 
back, and at every fifth ring enlarges; out of this enlargement 
branch on each side two smaller vessels. These knots are, in 
reality, hearts. The muscular system of the creature observes the 
there is a group of muscular fibres attached to 
same regular order 
every fifth ring; and the reproductive organs also observe the same 
regularity. Thus a fragment of the leech which is contained by 
five rings, possesses a ganglion, a heart, a stomach, and a muscular 
and reproductive system; that is, a complete organism, sufficient 
for a distinct individual. In other words, the leech is a series of 
symmetrical animals joined together. We cannot say that it is a 
simple, but a compound annelid. 
The common earth-worm, the centipede, and other annelids, 
are formed on the same principle. We have often said that Nature 
progresses not by leaps, but by very graduated steps; and that, 
between one organisation and another, there is generally an inter- 
mediate stage. The polypiers are associations of innumerable 
animals, actuated by a common hope of life, and bound together 
by indissoluble bonds. But most animals have a solitary existence, 
and are complete in themselves, having no continued connection 
with any of their race. The leeches fill up the space between 
the two—polypes, which have quitted their polypidoms, and have 
joined themselves into one body. 
The leech is the type of this mode of construction, but all 
the annelids bear. some resemblance to the arrangement. In the 
year 1826, the name Zoonites was bestowed upon the class. The 
zoonites do not necessarily adhere to the plan exhibited in the 
structure of the leech, having their organs situated in a space 
included in five rings; but in some, the interval is measured by 
four, in others by three, rings, and so on, until some have in every 
ring a complement of organs. When, as in the worm or leech, 
the rings are arranged in one line, the animals are classed as 
Articulata. The linear composition is not universal: the star-fish 
and the pyrosoma are constructed on this principle, and yet 
they are not articulata. The pyrosoma, of which we treated in 
Chapter XVIII, are the connecting link between the associated 
polypes and the articulata. 
It is often asked, Why is a quadruped instantly killed when the 
