12 PISCES. 



CLASS 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, 

 Chimceras, 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, Telggstomi, and Dipnoi. 

 The last group (Ceratodus, Lepidosireri) has well-developed lungs 

 and the paired fins flipper-like. It forms a connecting link be- 

 tween the Ganoidei and the BatracJiia. 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: embrvo 

 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. 



b. Giiropenings 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.) SELACHII, page 14. 



