70 LOWER VERTEBRATES. 



nerves coalesce to form a chiasma, as in ttio higher vertebrates, the olfactory lobes are 

 lartre, and the olfactory nerves are usually long, dilating into large ganglionic masses 

 at the base of the nasal sacs. The eyes are sometimes provided with a third eyelid — 

 the nictitating membrane — much like that of birds. The labyrinth of the ear (except 

 in Holocephali) is completely enclosed in cartilage. In the Squali the semicircular 

 canals are normal, but in the Rai» they are circular, and each opens into the vestibule 

 by a separate caual. The ear communicates with the exterior by a canal opening on 

 the top of the head and partially covered by a valvular flap. 



The paired male reproductive organs are much like those of the higher vertebrates, 

 and the ovaries are usually paired ; in some, however, but a single ovary occurs. The 

 eggs are few in number, and large, a great amount of food yolk being present. 

 The Holocephali, some llaiee, and a few of the sharks, are oviparous, and lay eggs 

 enclosed in horny capsules, which frequently are prolonged in long horns at the 

 corners. Most of the sharks are viviparous, and, in a few, a rudimentary placenta is 

 formed. In the others, judging from the increase in size of the young during foetal 

 development, it is probable that some nourishment is received from the mother, though 

 no connection exists between them. 



Owing to the ease with which the eggs are procured, their size, and the morpho- 

 logical itnj)ortance of the Elasmobranchs, the embryology has been extensively studied ; 

 the monograph of Balfour easily occupying the first place. The eggs undergo a mero- 

 blastic segmentation, and soon a nearly circular blastoderm rests on one side of the yolk. 

 At one edge of this a swelling appears, and, with growth, this increases in length, and 

 forms the first rudiments of the embryo. As development proceeds, this embryo is 

 elevated above the rest of the yolk, which becomes covered by the spi-eading blasto- 

 derm, or the yolk sac, as it is called. In this yolk sac, blood vessels are developed, and 

 the yolk is gradually transformed into blood for the growmg young. In the living 

 embryo, the circulation of the blood through this vitelline system can i-eadily be seen. 

 At an early stage the notochord becomes separated from the hypoljlast, then the muscle 

 plates (myotomes) appear, the brain becomes divided into its three parts, and the 

 epiblastic involutions for the eyes begin. Then that for the ears occurs, and the vis- 

 ceral clefts are formed. The heart now begins to bend and to beat. Soon the bran- 

 chial filaments protrude from the visceral clefts, and the continuous median fin is 

 differentiated. During this time the yolk is gradually growing smaller, and the 

 elements of the paired fins are forming. From this stage the development into the 

 adult condition is regular, and the embryo is born or hatched, as the case may be, be- 

 fore the whole of the yolk sac is absorbed. 



The Elasmobranchs first appear in rocks of upper Silurian age, and in the carboni- 

 ferous and Permian rocks their remains are numerous, though they consist chiefly of 

 teeth and ichthyodohirites, the determination of the position of which in a systematic ar- 

 rangement is almost impossible. In the mesozoic rocks both sharks and skates are found. 

 The older .sharks and skates were mostly provided with pavement teeth, indicating 

 a food of molluscs and crustaceans ; in the carboniferous, the trenchant teeth began 

 to predominate, indicating a more active and a more jiredacious nature in their 

 possessors. In the later rocks the fossil teeth are very abundant, especially at certain 

 localities. 



The Elasmobranchs are divided into two groups, the Holocephali and the Plagi- 

 ostomi. The former is the more generalized, and, as we shall see, forms what may be 

 called a connecting link between the various groups of lower vertebrates. 



