12 PISCES. 
Cuass E.— PISCES. (THE FISHES.) 
A “fish” in the popular sense is a member of any one of the 
three classes of aquatic or fish-like vertebrates, the groups here 
designated as Leptocardii, Marsipobranchii, and Pisces. But the 
Lancelets and the Lampreys differ so widely from the other groups 
that we must exclude them from consideration as fishes. Many 
writers go still further and remove from the Pisces, the Sharks, 
Chimeras, and Dipnoans, but for our present purposes all these 
may be referred to the same class as the true fishes, or Teleosts. 
The Pisces or “ Fishes” may then be defined as cold-blooded ver- 
tebrates adapted for life in the water, breathing by means of gills 
which are not purse-shaped, but attached to bony or cartilaginous 
gill arches; having the skull well developed and with a lower jaw; 
with the limbs present and developed as fins, or rarely wanting 
through atrophy; with shoulder girdle present, furcula-shaped, 
curved forward and with the sides connected below; with pelvic 
bones present; having the exoskeleton developed as scales or bony 
plates or horny appendages, sometimes obsolete, and with the me 
dian line of body with one or more fins composed of cartilaginous 
rays connected by membrane. The existing representatives of the 
class Pisces may be conveniently divided into four subclasses: 
Selachii or Elasmobranchii, Holocephali, Teleostomi, and Dipnot. 
The last group (Ceratodus, Lepidosiren) has well-developed lungs 
and the paired fins flipper-like. It forms a connecting link be- 
tween the Ganoideit and the Batrachia. As there are no North 
American species of Dipnoi, the group needs no further men- 
tion in this work. 
Subclasses of Pisces. 
a. Gills not free, being attached to the skin by the outer margin. Ova few 
and large, impregnated and sometimes developed internally: embryo 
with deciduous external gills; membrane bones of head undeveloped, 
except sometimes a rudimentary opercle; skeleton cartilaginous; skull 
without sutures; tail heterocercal; ventral fins abdominal; male with 
large intromittent organs or claspers attached to ventral fins; skin 
naked or covered with minute rough scales, sometimes with spines; no 
air-bladder; arterial bulb with three series of valves; intestine with a 
spiral valve; optic nerves united by a chiasma; cerebral hemispheres 
united. 
8. Gill openings slit-like, 5 to 7 in number; jaws distinct from the skull 
joined to it by suspensory bones; no membrane bones; teeth distinct, 
(Sharks and Skates.) . . . 2 1 © © © © « SELAcnn, page 14. 
