S20 CAIITILAGINEI. PETROMYZONID.E. 
being cartilaginous instead of bony ; in the pre- 
sent Family the latter distinctive mark gradually 
disappears^ the spine in the highest forms being 
traversed by a single tendinous cord^ filled in- 
ternally with a mucilaginous fluid, without con- 
tractions and enlargements, so that the vertebrm 
are reduced to cartilaginous rings not easily dis- 
tinguishable from each other, and, indeed, not 
cartilaginous through their whole circle,” (Cuvier); 
— ^while in the lowest forms (Amphioxus)^ it is 
reduced to a simple cartilaginous column or 
thread, flexible, transparent, and scarcely to be 
distinguished from the horny pen enveloped in 
the flesh of some of the Mollusca. Hence it 
has been disputed whether these minute crea- 
tures have a right to a place among Verte- 
BRATA ; though the preponderance of opinion, 
founded on dissection and comparison of various 
organs, is in favour of such a position being 
assigned to them. 
What we have further to say must be consi- 
dered as applying principally to the more de- 
veloped members of the Family. They have no 
pectorals nor ventrals ; but foldings of the skin 
above and below the hinder parts of the body 
serve the purpose of dorsal, caudal, and anal ; 
destitute, however, of supporting rays. The form 
is long, slender, and cylindrical, much resembling 
that of a worm ; the mouth consists of a circular 
fleshy lip, with a cartilaginous ring supporting 
it. The gills are not comb-shaped fringes, but 
form sacs or pouches, by the union of two oppo- 
site ones along their edges. 
These humbly organized members of the great 
Vertebrate division of animated beings are but 
