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Whether an animal having a shell must naturally be enclosed inside the shell.
Whether animals having shells differ in shape, number, and substance.
Further, one asks whether shellfish-animals (or those having shells) must naturally be enclosed inside their shells.
1. It seems not. A shell in bloodless creatures is just like bone in those having blood. But bone lies within the flesh and not outside of it. Therefore, this will have to be so for a shell.
2. Moreover, every fluid naturally seeks an external location. But the flesh of such animals is fluid and watery and will naturally seek an external position; therefore, etc.
Related to this, one asks about the difference in shells with respect to shape, number, and substance.
To the first, one must reply that moisture is, naturally, poorly bounded by its own boundary and well bounded by another's. But the flesh of some animals is very moist, and this is why they naturally require something solid on the outside as their boundary and to retain and preserve them. But the flesh of some animals is sufficiently bounded by a fairly hard, viscous skin, and this is why something solid that is internal is adequate for them upon which their flesh is established and through which it is sustained. Examples include perfected animals such as the cow, the human, and others of this sort, and this is why the crab, oysters, and others of this type have external shells, because of their softness. But the genuses of cephalopods [malakie] have something that is hard or shell-like inside for its hardness, and are fleshy on the outside.
1. On to the arguments. To the first, one must reply that flesh has a different disposition in various animals, and that is why it is sufficient for some to have something solid that is internal while in others something solid that is external is required.
2. To the other argument one must reply that because moisture naturally seeks an exterior location, animals that have soft flesh necessarily require a hard exterior by which they may be bounded and conserved.
To that which is asked in relation to this, one must reply that the material of the shell is a viscous humor and the agent is hot, dry, and combustive. And when this humor is very viscous and cannot be easily divided, but rather it is uniformly dense, muddy, and mulcidus throughout, then a shell is generated for the whole animal, as is seen in the tortoise and snails owing to the uniform character of the material. When, however, the viscous humor is not so uniform but can be divided, two or more shells are generated through the heat, depending on the requirements and species of the animal and on the divisibility of the material. If the heat is very sharp and penetrating and the humor is very viscous with some earthiness mixed in, then lines or bumps or rough places appear on the shell because, in a place or part to which the heat and the humor are driven, there appears a bumpiness and a roughness, and in a place from which it is driven, there appears a concavity. But when the heat is weak and the humor is oily, then the shell is soft; but when the humor is very earthy with a burning heat, then the shell is as hard as a rock, as in the case of the snail, which is poisonous, because it always lives in a marsh and in mud and not in the sea. Thus all of the differences in shells are caused either because of the humor or vapor, or because of the material, or because of the heat acting in them, and because the humor of the sea is very viscous owing to its saltiness and is bitter from its brackishness. This is why shells or shelled animals are more often generated in the sea than in other places. They are generated less often in other places because the salt content of the sea makes for the dryness of their shells, etc.
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