CLASSIFICATION OF THE ANIMAL KINGDOM. 207 



forming a powerful lever on which the entire muscular system can act. 



It will not be necessary in a mere summary like this to enumerate all 

 the various parts composing the skeleton in the different classes; suffice it 

 to say that they all, more or less, accord with one great fundamental type, 

 but so endless are the modifications and supplimentary processes met with 

 in the different tribes, that no two genera are found exactly alike, and the 

 ideal model of a true perfect skeleton, according to philosophy, does not 

 exist in Nature. There are other organs also met with in the Vertebrate 

 world, which we have not yet seen in even a modified state, such as the 

 spleen, the pancreatic glands, and the Vena Portae, etc., which will be 

 glanced at as we proceed. The first great class is that of Fishes, animals 

 which, though they possess most of the organs typical of the highest tribe 

 of vertebrate animals, yet in many respects are still found associated with 

 the Cephalopods. In many of them, for instance, the skeleton, instead of 

 presenting the usual osseous structure, is still of a soft cartilaginous nature; 

 moreover in the structure of their scales, whether flat, as in the Perch, or 

 raised and thorny, as in some of the Skate tribe, or in whatever other 

 condition they may be found, they are still in their nature and mode of 

 secretion exactly like the shells, whether internal or external, of molluscous 

 animals. The respiratory organs of fishes are a series of pectiniform bran- 

 chial fringes supported on bones, and are situated on each side of the neck, 

 and respiration is effected by the continual suction of the surrounding 

 medium through their mouths, which passes by these branchiae or gills, and 

 so effectually aerates the blood. Their muscular system throughout is very 

 highly developed, and whether as an agent of locomotion or otherwise, is 

 capable of exercising very considerable force. Their mouths are frequently 

 very different in shape and structure; some are quite smooth internally, 

 but, generally speaking, they are more or less armed with teeth and dental 

 plates. To the mouth succeeds a short oesophagus, and generally capacious 

 stomach, as in the Cephalopods, terminating in osseous fishes in a simple 

 intestinal tube; but in those whose skeletons are of a cartilaginous nature, 

 the intestine is capacious and spiral. The liver is always of a large size, 

 and generally contains a quantity of oil, and the biliary ducts open into 

 the intestine. There are no salivary vessels in fishes, but they are provided 

 with pancreatic glands, by which a fluid of a salivatic nature is secreted, 

 and poured into the intestine. In these animals we also find for the first 

 time another set of vessels, called the Vena Portae, for separating the bile 

 from venous blood, that is, blood flowing from the branchial organs to the heart, 

 instead of arterial blood, as in all previous orders; also kidneys, which are 

 very voluminous, and a vascular organ, called the spleen, for converting 

 arterial into venous blood before its transmission into the liver. 



There is also another set of vessels met with in fishes, and peculiar to 



