8 INTRODUCTION. 



viduals of the same species ; but in some classes the individuals 

 are at once males and females. In this case these beings are 

 termed androgynous. Sometimes the individuals possess both 

 sexes, like the greater number of vegetables, and they are then 

 called hermaphrodites. 



The animals which have the sexes separate differ also among 

 themselves. Those are termed oviparous in which the germ 

 of the young individual is separated from the parent for a time 

 before birth under the form of an egg. Viviparous animals, on 

 the contrary, are those in which the young are nourished in an 

 organ termed the uterus, and are not excluded from the mother 

 till they have taken the form which they afterwards preserve. 



Other modifications are noticed among the oviparous animals, 

 or those which deposit eggs. In some the egg is impregnated 

 within the animal, and then the shell or covering is generally 

 solid or corneous. In others, such as fishes, frogs, some insects, 

 and many mollusca, the impregnation of the ovum does not 

 take place till after extrusion. Two remarkable circumstances 

 have been further observed among oviparous animals. The one 

 is, that in some species the ova are not truly excluded but hatch- 

 ed in the parent animal, who thus preserves the imperfect beings 

 till they have acquired the requisite solidity for being deposited 

 in a place adapted to their further developement. These species, 

 which are met with in very different classes, are termed ovo- 

 vivlparous. The other singular fact to be noticed in regard to 

 oviparous animals is, that in a very great number of species the 

 young when hatched have neither the form, the structure, nor 

 the manners of the parent animal, and many live in altogether 

 a different medium. These animals undergo in the course of 

 their limited existence many organic transformations or succes- 

 sive metamorphoses. Such in particular are the frogs and con- 

 nected genera, and the whole class of insects. 



The moving power is another characteristic of animal organi- 

 zation. It is seated in the muscular fibre, which is formed of 

 filaments of excessive tenuity, capable of contraction, and of 

 moving the parts upon which they are fixed. These fibres are 

 distributed over the body, and produce all its exterior and inte- 

 rior motions. When they are united in a bundle, of which the 

 mass co-operates in the same action, this bundle is termed a mus- 



