ON ENTOZOA. 549 
the articulations are so much separated, that they seem to form 
a sort of chain, which breaks with the greatest facility. 
Never, or scarcely ever, are any appendages observable on 
each side. The worms, in fact, are easy to be recognized by 
this great simplicity in the external form. 
Their organization, too, in general, is but little complicated, 
and often, in consequence of their smallness and the trans- 
parence of their tissues, it may be observed through the 
teguments. It is particularly when they are young, and 
living, that this transparence is almost perfect. 
The external envelope, or skin, is almost always confounded 
with the muscular substratum, which serves for locomotion. 
The dermis is consequently not distinct; it is always very 
soft, of a nature almost mucous, and we cannot distinguish 
above it, any other of the parts of the skin, such as they have 
been analyzed in this envelope, in superior animals. ‘Thus, 
neither papilla, nor epidermis can be recognized. ‘The vascu- 
lar net-work alone is sometimes tolerably developed, and we 
may remark there a pigmentum sufficiently marked, at least 
in some species; but all that are parasites in the intestinal 
cavities, or parenchyma of animals, are constantly white, 
unless they should be filled with some colouring matter, 
which may be perceived, in consequence of the transparence 
of their exterior envelope. 
In a very small number indeed have any indications been 
observed of special organs of sensation. 
The apparatus of locomotion consists solely in the muscu- 
lar stratum which doubles the skin, and the intestine itself. 
We cannot even say that these animals have true muscles. 
The muscular stratum is merely divided into eight longitu- 
dinal bands, by the dorsal, ventral, and lateral lines, which 
are themselves composed of interrupted fibres. In the vesi- 
cular species, where the body terminates by a bladder, the 
muscular fibres radiate all around its parietes. 
