320 CARTILAGINEI. PETROMYZONID^. 



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 vertebrae 

 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 

 Vertebraie division of animated beings are but 



