VERTEBRATA : GENERAL CHARACTERS. 457 



under the wholly abnormal condition of spending their exist- 

 ence in darkness (such as the Proteus anguinus of the caves of 

 Illyria). Smell, hearing, and taste are probably rarely, if ever, 

 altogether absent in Vertebrates ; though in many cases their 

 organs are very rudimentary. Touch, or " tactile sensibility," 

 is usually possessed to a greater or less degree by the entire 

 surface of the body ; but the sense of touch is generally localised 

 in certain particular parts, such as the appendages of the mouth, 

 the lips, the tongue, or the digits. 



In all Vertebrata without exception reproduction is carried 

 on by means of the sexes, and in all (except in some of the 

 Serranidce among the Fishes) the sexes are in different indivi- 

 duals. No vertebrate animal possesses the power of reproduc- 

 ing itself by fission or gemmation ; and in no case are compo- 

 site organisms or colonies produced. Most of the Vertebrates 

 are oviparous that is to say, the ova are expelled from the 

 body of the parent either before or very shortly after impreg- 

 nation. In other cases, the eggs are retained within the body 

 of the parent until the young are hatched, but no direct con- 

 nection is formed between the fcetus and the mother, and in 

 these cases the animals are said to be ovo-viviparous. In other 

 cases, again, not only is the egg hatched within the parent, but 

 the embryo is retained within the body of the mother, from 

 whom it receives nourishment by direct vascular connection, 

 until its development has been carried out to a greater or less 

 extent ; and these animals are said to be viviparous. 



Many vertebrate animals possess an exoskeleton, formed by 

 a hardening of one or other layer of the integument. The 

 integument is composed of two layers an external non-vascu- 

 lar "epidermis," and a deeper vascular "dermis" and the 

 exoskeleton may be formed by the deposition of horny matter, 

 or of salts of lime, in either or in both of these. The epider- 

 mal exoskeleton is always horny, and, when present, is gener- 

 ally in the form of hairs (Mammalia}, feathers (Birds), scales 

 (serpents and many lizards), or plates (Chelonians). The 

 horny sheaths of the jaws in Birds and some Reptiles, the 

 outer covering of the horns in some Mammals, the hoofs, 

 claws, and nails of Mammalia, are likewise epidermic. The 

 dermal exoskeleton may be either horny or bony ; and good 

 examples of it are to be found in the scales of fishes, the bony 

 scutes of the Crocodiles, and the armour-plates of the Arma- 

 dillos. 



DIVISIONS OF THE VERTEBRATA. The sub-kingdom Verte- 

 brata is divided into the five great classes of the Fishes 

 (Pisces), Amphibians (Amphibia), Reptiles (Reptilid), Birds 



