348 CHITo^’ID^. MOLLUSCA. Ciutonid^. 
to a considerable size, as the Chiton squamosus, &c. (see 
fig. annexed); these are eagerly devoured by the negroes, 
who call them beef. The thick fleshy foot is cut away 
from the living animal, and swallowed raw, while the 
viscera are rejected. The Chitons deposit their eggs, 
closely united in clusters, on stones, each egg being 
inclosed in a thick, vesicular, folded envelope. The 
young, when first hatched, have no shell ; that covering 
not being developed until some time after. The back 
of the newly hatched creature, however, is marked with 
seven cross furrows, and between them, the first rudi- 
ments of the shell make their appearance in the form 
of close granulations. As the animal increases in size, 
the furrows become more distinctly separated, and the 
first shell is then seen in the form of seven narrow bands 
with irregular margins. In a family containing upwards 
of two hundred species, it may easily be imagined the 
varieties in form, &c., must be considerable. The 
Chitons accordingly have been divided into no fewer 
than twenty-three different genera, and in the most 
recent arrangement of these, Dr. Gray has selected, as 
characters, the forms of the plates of insertion of the 
various valves. We must refer to his “Systematic 
Arrangement” for the details of the various genera. 
We will content ourselves in this place with merely 
stating, that they may all be ari'anged in two groups or 
sub-families depending upon the structure of the mantle. 
The true Chitons, the Chitornnce of Adams, have the 
mantle simple, and without pores on it furnished with 
tufts of bristles or spines. The Cryptoplacince of 
Adams, the second group, have the mantle furnished 
with pores which are beset with subulate, flexible 
bristles, or with tufts of bristle- like calcareous spines. 
Ghoup I.— Chitonin^. 
This group is by far the most numerous, and the 
edge of the mantle is variously covered by scales or 
calcareous thick bristles, like spines ; or it is smooth 
and naked. In the genus Chiton, the mantle margin 
is covered with regularly disposed, imbricate, smooth, 
polished scj^les. — (See fig. annexed of Chiton squa- 
mosus). 
In the genus Acanthopleuka, the mantle margin is 
covered with uniform shelly spines or elongated scales ; 
and in the genus Tonicia, the mantle margin is naked. 
smooth, and covered with a hard shining skin beneath. 
In all these the exposed part of the valves, or the part 
not covered by the mantle of the animal, is broad and 
band-like ; but in the curious genus Cryptocliiton, the 
Fig. 227. 
valves are entirely covered by the mantle, and this is 
set all over with numerous close, uniform tufts of cal- 
careous spicula, which in a dry state resemble at first 
sight particles of saw-dust. 
Gkoup II. — Cryptoplacika:. 
This group contains fewer species, and some of these 
present a good deal of similarity to annulose animals, 
from the smallness of the exposed portion of the shell, 
and the tufts of spines along the edge of the mantle. 
In Plaxiphora the pores of the mantle are furnished 
with subulate, flexible bristles. In Acanthochetes the 
pores are furnished with tufts of bristle-like calcareous 
spines, which are generally large and often iridescent. 
The exposed part of the valves is moderately broad. 
In Chitonellus., the mantle is covered with short, 
crowded, calcareous spines, and the pores on the sides 
are small, with similar spines; but the exposed portion 
of the valves is rather small, and the body of the animal 
is elongate, subcylindrical, and the back is convex. It 
is this peculiar form of the animal that causes them to 
resemble so much some of the Annelids, 
Sub-class II.— -HETEEOBRAN CHIATA. 
In this sub-class of Molluscs, the gills are not, as in 
the former sub -class (see p. 186), inclosed in a special 
cavity, but are more or less exposed on the back, or 
at the sides towards the hind part of the body. Their 
respiration, however, is aquatic as in the ctenobran- 
chiate orders; and the animals are hermaphrodite, the 
male and female organs being found in the same in- 
dividual. 
The number of species contained in this sub-class is 
very considerable, though certainly inferior to those of 
the preceding; and many of the genera have no external 
covering or shell in the adult state. The young or larvae, 
however, are in all of them provided with a spiral shell, 
the aperture of which is closed by an operculum. TJieir 
head is furnished with two lobes or fins, fringed at the 
margin with vibratile cilia, which serve as organs of 
locomotion, but which disappear as they reach the 
mattire state. 
