352 LOWER VERTEBRATES. 



during the tedious process of swallowing, when, too, the tracheal air sacs and the pos- 

 terior reservoir come into play. 



The ovaries and testes are paired ; the right, however, is often the larger and 

 placed in advance of the left. Tlie male lias a pair of introinittent organs, which are 

 grooved, and, when in use, are everted, like the finger of a glove. Some species have 

 these organs armed with sharp spines or hooks, anticipating the special development 

 of some of the rodents. No urinary bladder is found in Ophidia. 



The eggs of serpents are oblong in form and covered by a leathery envelope, for 

 the rupture of which the embryos are provided with an egg-tooth, a special develop- 

 ment like that of the chick. The eggs, whose embryos are well advanced before 

 deposition, ai-e ordinarily left to care for themselves, though the pythons con- 

 tinue to protect them, winding their body around, and, with their feeble heat, incubat- 

 ing them. In some forms, especially the members of the Solenoglypha and some of 

 the Proteroglyphs, the eggs hatch in the oviduct, an occurrence which may happen in 

 snakes ordinarily oviparous, and the young are thus born. The classification herein 

 adopted arranges all the members of the order Ophidia in four sub-orders : Opotero- 

 donta, with non-distensible mouth, facial bones immovably connected, teeth only in 

 one of the jaws, and the posterior limbs as rudiments; Colubriformia, having teeth in 

 both jaws, no fangs, and including all the more common harmless snakes ; Protero- 

 glypha, poisonous snakes, with large, permanently erect, grooved fangs, which are 

 placed anteriorly in the upper jaw, and are usually immediately followed by ordinary 

 teeth ; Solenoglypha, with the perforated fangs unaccompanied with ordinary teeth 

 and capable of being depressed. 



Sub-Order I. — OPOTERODONTA. 



The first sub-order inehides those forms which, because of their imperfect eye- 

 sight (rendered so by the odd arrangement of the lateral cervical plates, which so 

 cover the eye as to I'ender vision extremely indistinct), are popularly known as ' blind- 

 worms.' They are provided with a narrow, non-distensible mouth, and the bones of 

 the head are more firmly united together than those of other ophidians. The solid, 

 hooked teeth are only in one jaw (upper or lower) and the 

 body is terminated by a short tail. Posterior limbs are 

 sometimes present as rudiments. The larger number of 

 species Ijeloiig to the genus TyiMops. 



The Typiilopid^ are very generally distributed over the 

 F.G. 209.- Head of TypUop,. ^^^^^ portions of the globe, four species being found in 

 our country north of Panama. To the naturalist they are most interesting forms 

 and are eagerly sought after. They are the lowest as well as the smallest 

 ophidians, — an ordinary earth-worm is gigantic when compared with some. Their 

 short muscular body, designed for underground tunnelling, rudimentary eyes, and 

 peculiar dentition, are special points of interest. The genus Typhlops has the 

 body covered with small imbricate scales ;■ the tongue forked ; a pair of scarcely 

 discernible eyes ; and the lungs unequal in size. The head may be obtuse, 

 depressed, or in some species drawn out into a horny tip ; the tail may also be en- 

 sheathed in horn, as is the case with TypJdops philippinus. The upper jaw is toothed. 

 Many species of this genus inhabit British India, where, after showers, they come- 

 above ground and are very active. 



