280 THE WORLD OF THE SEA. 
head is severed from the body, and yet a leech will live for a whole 
year after it has been mutilated? The answer is simple: because 
a quadruped has only one centre of its nervous system, one centre 
of life—the brain—and if that be severed from its trunk, death 
must follow; whereas, in the case of the zoonite, that particular 
nerval centre only is destroyed which has sustained the injury— 
the rest continue their action unimpeded. 
This idea of the zoonite construction has, of course, met with 
much opposition. In the first place, it was objected that the two 
suckers, the oval and the ventral, were so distinct that they could 
not possibly be said to have the same construction: one had eyes 
and mouth, the other one only the anal orifice. Yet these are 
so alike that the same name is applied to each of them, and in 
the sea-leech they can hardly be discriminated. Indeed, Baster, 
an eminent zoologist, actually confounded the two. Again, it is 
urged that many of the zoonites have absolutely only one organ 
of a particular kind, hence their several parts cannot possibly be 
said to have a like organism. Take, for example, the Planarta— 
aquatic animals which inhabit both salt and fresh water—they 
are closely allied to the leeches. They have but one orifice to 
their digestive cavity, and in its neighbourhood is a flexible horn, 
by which the creature seizes its prey, and introduces it into its 
stomach. Here digestion is completed, and the rejected refuse is 
taken out again by the horn. The opponents of the zoonite 
theory cite such an example as this, triumphantly asking if it can 
be still asserted that the planaria is made in sections which are 
exactly alike. If, however, the creature be cut in half, either above 
or below the digestive pouch, each part will continue to live—one 
having the stomach near its extremity, the other being destitute 
of that very essential organ. However, in a short time a white 
spot appears in the centre of each fragment, which gradualiy 
extends until an orifice opens as the mouth of a new stomach, 
the old one, in the fragment above alluded to, closing. There is 
a time when the planaria possess two stomachs—the new one and 
the old one. This fact is a sufficient proof to us that, although 
the creature has only one stomach, yet each of the symmetrical 
parts of its organism is prepared to develop one when necessity 
