582 PISCES. 



SUBKINGDOM VERTEBRATA. 



THIS is the highest division of the Animal Kingdom, and, until within the 

 last twenty years, the essential character upon which the subkinglom was based 

 was the possession of a bony or cartilaginous internal skeleton, having a spine or 

 vertebral column. Since that time the class known as Tunicates, or Ascidians, 

 which have no bony skeleton, has been referred to it; and the Amphioxus lanceo- 

 Jatus, a little, slender, transparent creature, having only a gelatinous cord, no 

 brain cavity, and colorless blood, which was regarded as the lowest type of fishes, 

 and had been elevated into an order called Leptocardia, is now taken out of the 

 Class Pisces and referred to a separate class called Acrania. The Classes Tuni- 

 cata and Acrania are not, however, known in Palaeozoic rocks. Another class, 

 called Cyclostomata, consisting of long, cylindrical, worm-like bodies, with a tough 

 skin, destitute of scales, pectoral and ventral fins, but having a fin at the extremity 

 of the body without any rays, and having a cartilaginous skeleton, and which in- 

 cludes the various species of lampreys that inhabit fresh water, and are also found 

 in the ocean, and which have generally been regarded as an order of fishes, is 

 unknown in Palaeozoic rocks. Some authors would, however, place the Conodonts 

 in this class ; but if they do not belong to the Annelida, then there are stronger 

 reasons for believing they belong to Crustacea than for thinking they should be 

 referred to the Cyclostomata. 



No Palaeozoic fossil from an animal as highly organized as the lowest mammal 

 or a bird has ever been found. The fossils are confined to the lowest organizations 

 of fishes, batrachians, and reptiles. The lower forms of fishes have only a cartilag- 

 inous cord, resembling the embryonic state of fishes having an osseous skeleton. 

 In higher forms the spine consists of bony vertebrae, united in such manner as to 

 allow flexibility and strength by reason of attaching muscles, and also to protect a 

 spinal nerve that passes through it. 



CLASS PISCES, OR FISHES. 



The Class Pisces has been divided into four subclasses Ganoidea, Selachia, 

 Dipnoa, and Teleostia. The Teleostia have been divided into eleven orders, and 

 these into twenty suborders. This subclass embraces a very large majority of 

 the living fishes. All of them have a complete bony vertebral column and skull. 

 Nearly all edible fishes belong to this subclass. Many have a naked skin, but the 

 majority are covered with horny scales of various forms. When the scales are 

 smooth the fish are said to belong to the Cycloidea; when the hinder margins of 

 the scales are denticulated they belong to the Ctenoidea. Fishes have pectoral fins, 

 which are called the representatives of anterior limbs; and ventral fins, representa- 

 tives of posterior limbs; and also dorsal, caudal, and anal fins. This subclass is 

 unknown in Palaeozoic rocks. Agassiz divided the fish into four groups Cycloids, 

 Ctenoids, Placoids, and Ganoids based on the character of the scales; but as a 

 single fish has been found bearing two of these types of scales, and as it is now 



