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THE ANIMALS WITHOUT BACKBONES THE INVERTEBRATA. 



INTBODUCTION. 



Characteristics of Vertebrata Modifications Characteristics of the Invertebrata Various Distinctions among Themselves 



Habits Classification Intermediate Groups. 



THE animals, whether mammals, birds, reptiles, amphibians, or fish which have been described or 

 noticed hitherto in this work, have some parts of their construction which are similarly placed, and 

 fashioned after one plan. They have a series of bones, or vertebrae, forming the spinal column, 

 upon which the skull is placed ; and these structures separate the brain and its continuation the 

 spinal cord from the organs of digestion and respiration, and from the main organs of circulation. 



These animals constitute the great group of the animal kingdom, which is called the " Vertebrata." 

 They have red blood, and in some classes it is warm, and in the i-eptiles, amphibians, and fish it is cold. 

 They have an internal skeleton, and never more than two pairs of limbs, and these are modified to 

 meet the wants of the different classes, and in some instances they are more or less defective. One 

 side of the body has a general resemblance to the other, but different organs are found on opposite 

 sides within, in relation to the digestion and circulation, so that there is, generally speaking, a bilateral 

 symmetry. The development of the nervous system, and especially of the spinal cord, brain and large 

 nerves, is considerable even in the fish, and is increasingly great in the amphibians, reptiles, birds, and 

 mammals. The organs of special sense seeing, hearing, smelling, and of the sense of touch are 

 highly developed for the most part, and their possessors lead, sooner or later, independent lives, and 

 seek their food. 



Taking the great Apes as the highest of the animals we have noticed, and the Amphioxus as the 

 lowest, so far as the scale of construction is considered, they and the intermediate mammals, birds, 

 reptiles, amphibians, and fish are linked together by possessing many similarly arranged structures. 

 The mammals can be readily distinguished, but the simplest and lowest of them, the Monotremes, 

 have many points of anatomical resemblance with the reptilia and birds. The birds are linked on to 

 the reptilia of the past history of the globe, and the reptilia to the amphibia and fish. Moreover, these 

 last are closely allied by kinds which have structural arrangements that can hardly be definitely 

 said to be those of the amphibian or those of a fish. From the history of the past, it is gleaned that 

 these great groups, so interestingly linked together in the chain of natural classification, began to be 

 and appeared in the order of their present classificatory position. The fish and amphibia preceded 

 the reptilia ; birds came next, and then the lowest kind of mammal. Thus, by their possessing an 

 internal skeleton, a backbone, a skull, and limb-bones ; by their having the nervous and vegetative 

 systems separated, the one near the back (dorsal) and the other ventral ; by their classes being 

 connected by many common structures and by their geological history, the Vertebrata are a remark- 

 ably distinct and recognisable group. 



These general statements have only to be modified in a few instances. In some of the simplest 

 Vertebrata, that is to say, in some whose construction is less elaborate than in others, the spinal 

 column, with its succession of separate bones, is replaced by an elongated rod of cartilage which is 

 flexible, and to which certain membranes adhere. One membrane is folded above the rod (in the 

 swimming position of the fish Amphioxus, page 147), and envelops the spinal marrow, and 

 two others extend in the opposite direction and form and bound the cavity which contains the viscera. 

 This rod, or corda dorsalis, really supports the spinal nervous system, and separates, as in the other 

 Vertebrata, the nervous and vegetative organs. It exists in the early unborn or embryonic state of 

 all animals which have, when born, a series of jointed vertebrae forming a backbone, and it is 

 probable that the first fish that lived on the globe had this corda dorsalis or notochord only. 



In the Amphioxus there is no true brain-case, and the special senses of hearing and seeing 

 are at their lowest ebb, the ear being deficient, and the single eye is a mere mass of pigment, placed 

 on the nervous swelling at the fore part of the spinal marrow which represents the brain of other 

 Vertebrates. There is, therefore, no brain in this creature, and were it not for the cartilaginous 

 rod and the relative position of the nervous system and of the digestive and circulatory organs the 

 one above and the other below the rod the animal could hardly be called one of the Vertebrata. It 

 has colourless blood, it has 110 true jaws, and the mouth opens into a cavity which is used for the 



