XXIII] OPHIDIA 589 



mass by coiling themselves around it, and they then swallow it 

 whole. 



The hyoid, including under that name the remains of all the 

 hinder visceral arches, is vestigial, consisting of a single bone on each 

 side. This permits of the pulling of the glottis far forward between 

 the halves of the mandible when the animal is engaged in swallowing 

 its prey, this shifting of position being necessary to prevent choking. 



In the skull the brain extends forwards between the eyes and 

 there is consequently no interorbital septum. That this is a 

 secondary and not a primary state of affairs is shown by the fact 

 that the front part of the brain is protected at the sides by down- 

 ward extensions of the frontal and parietal bones, whereas the animals 

 such as the Urodela and Mammalia, where an interorbital septum 

 has never been formed, the side-walls of the cranium are constituted 

 by the orbitosphenoid and alisphenoid bones. It is curious to find 

 this absence of an interorbital septum in a family of limbless Lizards, 

 the AMPHISBAENIDAE. What relation, if any, it has to the snake- 

 like habits it is hard to guess. 



The two eyelids have coalesced to form an extra guard in front of 

 the eye, but there is a transparent portion in the lower one through 

 which the animal can see. The outer covering of scales is shed 

 periodically, half-a-dozen times every year or oftener, and replaced 

 by a new set formed by the activity of the ectoderm, and during this 

 process, since the covering of the eye is affected, the snake is blind. 



One lung is small, and the other (the right) greatly elongated, 

 the hinder part being quite smooth. 



The heart resembles that of Lizards both in structure and the 

 mode of distributing the arterial and venous blood. The differences 

 between the vascular systems of a Snake and a Lizard depend chiefly 

 on the absence of limbs and the correlated great development of the 

 vertebral column, ribs and their musculature as organs of locomotion 

 in the Snake. Thus the subclavian arteries are absent from the 

 right systemic arch, while the vertebral and caudal arteries and 

 veins are well developed. Another difference is that the left 

 pulmonary artery is very slightly developed, in connection with 

 the reduced condition of the left lung. 



Snakes are divided into many families, of which two are repre- 

 sented in Great Britain and three in the temperate parts of North 

 America. A rough classification would divide them according to 

 their habits into : (a) those which poison their prey, (b) those which 

 crush their prey, and (c) those which swallow their prey directly. 



