Their stomach is single, and the large intestine, as they are vegetarians, is large, and its termination the “cæcum” also, but it has no little worm-like appendage as in the Gibbons. No especial points have been noted in the muscular system, except the very curious fact that, although the bones of the thumb are so rudimentary, the muscles are all there except the one which principally bends it forward. As the activity of the Spider Monkey is marvellous, as they swing on and catch hold of boughs with great skill and energy, and as they display much intelligence, their brains ought to be well developed. Doubtless there is a great deal of movement in these long-limbed creatures which takes place like the walking of man, _i.e._, without direct thought, for we move our leg muscles, and all those which assist them in the act of walking, without a constant direction of the will. Just as man’s walking is thus said to be done automatically, so much of the swinging and progression of the Ateles is produced without direct exertion of the will. But it is evident that the Spider Monkey judges his distance, and very often considers whether such and such a bough will bear his weight, and uses exactly sufficient muscular exertion for what he requires. Moreover, there is a graceful co-ordination or mutual action of the muscles of the limbs, body, and tail to a common end in most of its movements which is evidently done by will. The movements of the tail are perfectly wonderful, and, indeed, so perfectly does it hold on, although the animal cannot see what this long slender organ is doing, that most children think there is an eye at the end of it. Directly the Spider Monkey rises on its hind legs, up goes the tail straight behind its back, and curves a little at the tip downwards: the delicate hairs stick out and feel the slightest touch or passage of air; and the least touch induces the last few joints to clasp hold. The animal will walk along and catch hold of things with its tail at every other step or so, and will change its hold in exact proportion to its rate of progression. All these movements necessitate clasping, unclasping, twisting, and a regular succession of efforts, and are not likely to be carried out except by an animal with a well-developed nervous system. Hence it has been a matter of some interest to compare the brain of Ateles with those of other Monkeys, and even with that of man. [Illustration: JAW OF THE SPIDER MONKEY.]