302 COMPARATIVE ZOOLOGY. 
the Lancelet, or Amphioxus. This lowest known Verte- 
brate is about two inches long, semi-transparent, and of 
worm -like form, and is found in the sandy bottom of 
many seas, especially the Mediterranean,” 
2. Marsipobranchs, as the eel-like Lamprey and Hag. 
They have a cartilaginons skeleton and sac-like gills, but 
no scales, limbs, or lower jaw, and only one nasal organ, 
all other Vertebrates having fwo. 
3. Teleosts, including all the common Fishes, having a 
bony endoskeleton and a scaly exoskeleton.’ The skull 
is extremely complicated; the upper and lower jaws are 
complete (whence the name of the order), and the gills 
are comb-like or tufted. The tail is homocercal; the other 
fins are variable in number and position. In the soft-fin- 
ned Fishes, the ventrals are absent, as in the Eels; or at- 
tached to the abdomen, as in the Salmons, Herrings, Pikes, 
aud Carps; or placed under the throat, as in the Cod, Had- 
dock, and Flounder. In the spiny-finned Vishes, the ven- 
trals are generally under or in front of the pectorals, and 
the scales ctenoid, as in the Perches, Mullets, and Mack- 
erels. 
4. Ganoids, distinguished by their enameled bony plates 
or scales. The endoskeleton is not completely ossified ; 
the ventral fins are placed far back; and the tail is gen- 
erally heterocercal. The gills are like those of the bony 
Fishes. This was one of the largest orders in old geo- 
logical history. The few modern representatives, as the 
Sturgeon, Gar-pike, and Polypterus, are essentially fresh- 
water. 
5. Elasmobranchs, having a gristly skeleton, and a harsh 
skin, called “shagreen.” The gill-openings are uncovered ; 
and the mouth is generally under the head. The ventral 
fins are placed far back; the pectorals are large, in the 
Rays enormously developed; and the tail is heterocercal. 
Such are the Sharks, Rays, and Chimeera. They are all 
