In the fourth book, Aristotle treats of the animals which are destitute of red blood. Of these, he says there are several genera: the mollusca, such as the cuttlefish, which is externally soft with an internal firm part; the crustacea, internally soft and covered with a firm integument, such as the crab; the testacea, internally soft and externally hard and solid, as the limpet and oyster. The insects form the fourth genus; and are distinguished by their being externally and internally formed of a hardish or cartilaginous substance, and divided into segments; some of them having wings, as the wasp; while others have none, as the centipede. He then gives a pretty full account of the cuttlefish and nautilus, treats of the crustaceous animals generally, and enters into details respecting the other two classes. After this he enumerates the organs of sensation, stating that man, and all the red-blooded and viviparous animals, possess five senses, although in the mole vision is deficient. He describes correctly the eye of that creature, showing that it is covered by a thickish skin, but presents a conformation similar to that of other animals, and is furnished with a nerve from the brain. He shows that although fishes have no visible organs of smelling or hearing, they yet possess both senses, and, in treating of this subject, states many interesting facts relative to the mode employed in catching dolphins. He also shows that insects have the faculty of hearing and smelling. The testacea, he says, besides feeling, which is common to all animals, have smell and taste; but he also asserts that some of them, the solen and pecten, are capable of seeing, and others of hearing. All viviparous quadrupeds not only sleep, but also dream; but whether the oviparous dream is uncertain; although it is plain that they sleep, as do the aquatic animals, fishes, mollusca, testacea, and crustacea. A transition is then made to the subject of sex, for the purpose of showing that in the mollusca, crustacea, testacea, and eels, there is no difference in that respect between individuals of the same species. The subjects of generation and parturition occupy the fifth, sixth, and seventh books. From the comparatively large space which he has devoted to the result of his inquiries in these departments, the minuteness with which he describes the phenomena presented by them in man and the domestic animals, and the accurate knowledge which he frequently exhibits, it may be inferred that they were favourite subjects with Aristotle. It is sufficient for our purpose to mention some of the cases in which he attained the truth, and others in which he failed.