Speaking generally, the Sanguinea differ from man and from one another in their parts, which may be present or absent, or exhibit differences in "excess and defect," or in form. Unity of plan extends to all the principal systems of organs. "All sanguineous animals have either a bony or a spinous column. The remainder of the bones exist in some animals; but not in others, for if they have the limbs they have the bones belonging to them" (Cresswell, _loc. cit._, p. 60). "Viviparous animals with blood and feet do not differ much in their bones, but rather by analogy, in hardness, softness, and size" (Cresswell, _loc. cit._, p. 59). The venous system, too, is built upon the same general plan throughout the Sanguinea. "In all sanguineous animals, the nature and origin of the principal veins are the same, but the multitude of smaller veins is not alike in all, for neither are the parts of the same nature, nor do all possess the same parts" (Cresswell, _loc. cit._, p. 56). It will be noticed in the first and last of these three quotations that Aristotle recognises the fact of correlation between systems of organs,--between limbs and bones, and between blood-vessels and the parts to which they go. Sanguineous animals all possess certain organs--heart, liver, spleen, kidneys, and so on. Other organs occur in most of the classes--the oesophagus and the lungs. "The position which these parts occupy is the same in all animals [sc. Sanguinea]" (Cresswell, _loc. cit._, p. 39). Unity of plan is observable not only in the Sanguinea, but also within each of the other large groups. Aristotle recognises that all his cuttlefish are alike in structure. Among his Malacostraca he compares point by point the external parts of the carabus (_Palinurus_), and the astacus (_Homarus_), and he compares also the general internal anatomy of the various "genera" he distinguishes. As regards Testacea, he writes, "The nature of their internal structure is similar in all, especially in the turbinated animals, for they differ in size and in the relations of excess; the univalves and bivalves do not exhibit many differences" (Cresswell, _loc. cit._, p. 83). There is an interesting remark about "the creature called carcinium" (hermit-crab), that it "resembles both the Malacostraca and the Testacea, for this in its nature is similar to the animals that are like carabi, and it is born naked" (Cresswell, _loc. cit._, p. 85). In the last phrase we may perhaps read the first recognition of the embryological criterion.