CHITONS. 
399 
characteristic of the different groups into which the Chitonidoa have been divided. 
The gills are placed on each side between the mantle and the foot, and vary in 
length. Adult chitons have neither eyes in their head nor tentacles, but the 
mouth is provided with a long radula, the teeth of which are arranged in a very 
peculiar complicated manner. The chitons are bisexual, but, like the limpets, 
destitute of certain functional organs. In the early stages the young chiton bears 
no resemblance whatever to its 
parent. This can be appreciated by 
observing the accompanying figures 
illustrating three stages in the 
development of Chiton marginatus, 
one of the dozen species found on 
the British coasts. The embryo {A) 
is a spherical body, of an inch in 
diameter, divided into two unequal 
parts, the dividing line being marked 
by a row of cilia, and a tuft of similar cilia being situated at the vertex of the 
smaller half. The eyes are visible upon the lower portion below the cilia. In a 
later stage (B and C ) the division of the back into eight sections is remarkable 
and unique in the Mollusca. At this stage the foot begins to develop, the forepart 
of the animal being ciliated. In later stages of the development the eyes and cilia 
disappear, the forepart shrinks up into the ridge surrounding the mouth, and the 
back develops the eight shelly plates. It has already been stated that the adult 
Chiton has no eyes. This is only true as regards the head of the mollusc, for 
Moseley made the discovery that certain forms have the shell studded with eyes, of 
which as many as eleven to twelve thou¬ 
sand sometimes exist in a single individual. 
They are not unlike those already referred 
to as present on the back of Onchidium. 
Chitons live principally on rocks and 
under stones at low water, or at moderate 
depths, but a few have been obtained as 
low down as two thousand three hundred 
fathoms. They are all marine, very slug¬ 
gish in their movements, and, if disturbed 
from their resting-place, roll themselves 
up into a ball like a wood-louse. The 
number of recent species is considerable, 
and they appear to range all over the globe. Fossil remains of certain forms have 
been found in most geological periods, since the Silurian. 
eyed chiton (Schizochiton incisus ). Anterior valve, 
with six rows of eyes (enlarged). 
Order Aplacophora. 
The molluscs of this order are somewhat worm-like, with a mantle enclosing 
the entire body, but not secreting a shell, and more or less studded with spicules. 
They are related to the chitons by certain points in their anatomy, especially with 
