406 LOWER VERTEBRATES. 



unprotected by lids; the higher forms have well developed lids; some, like the 

 skiiiks, having the lower so transparent that it performs the oiKce of a nictitating 

 membrane, though closely related forms may have it opaque and scaly. The keen- 

 sighted chameleons have the eye entirely surrounded by the lid, vision being obtained 

 through a central slit. 



The integument of lizards, though often provided with scales, is not invariably 

 so protected; the geckos and Amphisbsenas offering the most familiar exceptions. 

 The scales, when present, present considerable variety of structure, which is of use in 

 determining the several genera and species. These scales are often of considerable 

 size, especially when connected with the cutaneous expansions of the throat, back, 

 and sides. Along the inside of the thigh and across the abdomen the skin is not 

 infrequently pierced by duets leading from subcuticular glands. These openings are 

 called poi-es. 



Though the majority of lizards are oviparous, a few, like Anguis, /Seps, and P/jry- 

 ■nosoma, give birth to their young. With the exception of Heloderma, all are per- 

 fectly harmless so far as poison is concerned, and are generally of a most timid 

 nature. They are by far the most numerous, and present the greatest variety of 

 coloration in the typical countries. 



The classification herein adopted is the most natural, and is based on a thorough 

 study of the anatomical peculiarities of the order, a result of the labors of Pro- 

 fessor E. D. Cope and G. A. Boulenger. Twenty-one families are characterized, all of 

 which are treated to a moi-e or less extent. 



The first family, Geckoxid^, is easily recognized. It includes a number of the 

 lower developed lizards, which have the centra of the vertebris concave both ante- 

 riorly and j)osteriorly. They are further characterized by having the tongue short, 

 thick, and fleshy, the eyelids rudimentary, and the pupils of the large eyes generally 

 vertical and elliptical, a peculiarity which points to a nocturnal life. They are all plu- 

 rodont, and the head is broad and depressed ; the body is of moderate breadth, granu- 

 lar above, and covered below with small imbricate scales ; the tail is normally thick at 

 the base, and tapering, though it is so often broken off that it is generally somewhat 

 defoi-med. The limbs are stout, of moderate length, and the well-developed toes are 

 usually jjrovided with an adhesive apparatus, made up of a series of plates or disks, 

 by means of which the animals can run up a perpendicular wall or smooth tree. 

 Though nearly all are provided with claws, the sucking-disks are less perfectly 

 developed in the arboreal forms. The acri<l fluid secreted by the disks has given the 

 erroneous idea that the animals are poisonous. 



There has been observed in many geckos a peculiar pair of calcareous masses on 

 each side of the neck. These seem to vary in size with different individuals, in some 

 being entirely absent, while in others of the same species they may appear either as a 

 thin layer or as hard rounded masses. No dermal jiore has been discovered to con- 

 nect them with the exterior. 



The geckos are small in size, never exceeding fourteen inches in length, and are car- 

 nivorous ; destroying the larger insects and moths, and are to some extent canni- 

 balistic, eating their own young, and, what seems most surprising, they have been 

 observed to devour their own tail, an organ which they seem to regard as purely orna- 

 mental, to be dispensed with whenever the occasion demands. Among themselves 

 they are quarrelsome, and often fight over their prey. They are noisy at night,, 

 many being named, as is the gecko, from the peculiarity of their calls. 



