Secretary bird killing poisonous snakes, 121. Self-sacrifice, a law of life, 352. Sentinels placed by animals, 269. Shark _carrying remora_, 51; accompanied by pilot-fish, 52; eggs, 28, 42; external gills of embryo, 73. Sharks, history of the, 41; their structure, 28–30. Sheat-fish, fathers carrying eggs in the mouth, 65. Sheep and goats, range of, 274. Shell of tortoise, how formed, 98. Shields of the crocodile, 108. Shrike, a perching bird of prey, 172. Shrew and harvest-mouse compared, 218. Silver Pit, soles living in the, 63. Siren, a gill-breather, 80. Sirenia, name for sea-cows, 315. Skates, flat bodies of, 41; purse-eggs of, 42. _Skeleton, of bat_, 233; _bird_, 126; _wild ass_, 266; _lizard_, 103; _sea-lion_, 304; _snake_, 111; _tortoise_, 98; _whale_, 318. Skeletons of deep-sea fish, 49. _Skulls of rodent and insectivore_, 217. Skye, walrus seen off, 310. Slime on fish causing phosphorescence, 50. _Sloths hanging from a tree_, 200; structure and habits of, 206. Slowworm, a legless lizard, 107. Smooth hound, 28. Snake, affection in a, 349; casting its skin, 116; _common ringed_, 113; cup and ball joints of the, 112, 114; eggs, 116; enemies of the, 117; food of the common, 116; mode of swallowing prey, 114; _skeleton of a_, 111; coiling round prey, 117; nature of poison of, 119; poisonous sea-, 121. Snapping turtle, 101. Sole, history of its growth, 62; _young and old_, 61. Song of birds, how caused, 169. _Sparrow_, 125; _skeleton of_, 126. Spawning of lampreys, 18. Spermaceti in head of whale, 326. _Sperm whale_, 327; its structure and habits, 325; taking breath, 322. Spider monkeys, 246. Spinal cord, of lamprey, 17; of the lancelet, 12; of shark, 30; of vertebrata, 9. Spines, of hedgehog and porcupine, 222; of sharks, 29, 38. Spinous processes, of lizard, 103; of snake, 111; of tortoise, 98. Squirrels, enemies of the, 231; _flying_, 231; tree-loving rodents, 230. Stickleback father defending the young, 65. _Sticklebacks and their nest_, 65. Sting-rays, 42. Stomias, a deep-sea fish, 48. Storks, migrations of, 151. Struggle for existence, developing sympathy, 351; improving the herbivora, 282. Sturgeon _entering a river_, 31; first appearance of the, 41; _head and mouth of_, 32; structure of, 32. _Sucking-fish or remora_, 51. Sun-fish with large air-bladder, 53. Surinam toad, carrying young in hollows of the skin, 87. Swallow, a darter among the perchers, 172. Swans, range of wild, 142. Swifts, darting birds, 167; in East build nests with saliva, 167. Sword-fish, Xiphias, 55. Tadpole, _life and changes of_, 72–76; of salamandra atra, 81; summary of changes in the, 335. _Taguan or flying squirrel_, 231.