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  necessarily be of a corresponding symmetry. For the spherical shape

  has not the asymmetry of the disk-shaped body of the oysters. For in

  all these animals the head is central, but in the sea-urchin the

  so-called ovum is above [and symmetrical, while in the oyster it is

  only one side]. Now the necessary symmetry would be observed were

  the ovum to form a continuous ring. But this may not be. For it

  would be in opposition to what prevails in the whole tribe of

  Testacea; for in all the ovum is discontinuous, and in all excepting

  the sea-urchins asymmetrical, being placed only on one side of the

  body. Owing then to this necessary discontinuity of the ovum, which

  belongs to the sea-urchin as a member of the class, and owing to the

  spherical shape of its body, which is its individual peculiarity, this

  animal cannot possibly have an even number of ova. For were they an

  even number, they would have to be arranged exactly opposite to each

  other, in pairs, so as to keep the necessary symmetry; one ovum of

  each pair being placed at one end, the other ovum at the other end

  of a transverse diameter. This again would violate the universal

  provision in Testacea. For both in the oysters and in the scallops

  we find the ovum only on one side of the circumference. The number

  then of the ova must be uneven, three for instance, or five. But if

  there were only three they would be much too far apart; while, if

  there were more than five, they would come to form a continuous

  mass. The former arrangement would be disadvantageous to the animal,

  the latter an impossibility. There can therefore be neither more nor

  less than five. For the same reason the stomach is divided into five

  parts, and there is a corresponding number of teeth. For seeing that

  the ova represent each of them a kind of body for the animal, their

  disposition must conform to that of the stomach, seeing that it is

  from this that they derive the material for their growth. Now if there

  were only one stomach, either the ova would be too far off from it, or

  it would be so big as to fill up the whole cavity, and the

  sea-urchin would have great difficulty in moving about and finding due

  nourishment for its repletion. As then there are five intervals

  between the five ova, so are there of necessity five divisions of

  the stomach, one for each interval. So also, and on like grounds,

  there are five teeth. For nature is thus enabled to allot to each

  stomachal compartment and ovum its separate and similar tooth.

  These, then, are the reasons why the number of ova in the sea-urchin

  is an odd one, and why that odd number is five. In some sea-urchins

  the ova are excessively small, in others of considerable size, the

  explanation being that the latter are of a warmer constitution, and so

  are able to concoct their food more thoroughly; while in the former

  concoction is less perfect, so that the stomach is found full of

  residual matter, while the ova are small and uneatable. Those of a

  warmer constitution are, moreover, in virtue of their warmth more

  given to motion, so that they make expeditions in search of food,

  instead of remaining stationary like the rest. As evidence of this, it

  will be found that they always have something or other sticking to

  their spines, as though they moved much about; for they use their

  spines as feet.

  The Ascidians differ but slightly from plants, and yet have more

  of an animal nature than the sponges, which are virtually plants and

  nothing more. For nature passes from lifeless objects to animals in

  such unbroken sequence, interposing between them beings which live and

  yet are not animals, that scarcely any difference seems to exist

  between two neighbouring groups owing to their close proximity.

  A sponge, then, as already said, in these respects completely

  resembles a plant, that throughout its life it is attached to a

  rock, and that when separated from this it dies. Slightly different

  from the sponges are the so-called Holothurias and the sea-lungs, as

  also sundry other sea-animals that resemble them. For these are free

  and unattached. Yet they have no feeling, and their life is simply

  that of a plant separated from the ground. For even among

  land-plants there are some that are independent of the soil, and

  that spring up and grow, either upon other plants, or even entirely

  free. Such, for example, is the plant which is found on Parnassus, and

  which some call the Epipetrum. This you may hang up on a peg and it

  will yet live for a considerable time. Sometimes it is a matter of

  doubt whether a given organism should be classed with plants or with

  animals. The Ascidians, for instance, and the like so far resemble

  plants as that they never live free and unattached, but, on the

  other hand, inasmuch as they have a certain flesh-like substance, they

  must be supposed to possess some degree of sensibility.

  An Ascidian has a body divided by a single septum and with two

  orifices, one where it takes in the fluid matter that ministers to its

  nutrition, the other where it discharges the surplus of unused

  juice, for it has no visible residual substance, such as have the

  other Testacea. This is itself a very strong justification for

  considering an Ascidian, and anything else there may be among

  animals that resembles it, to be of a vegetable character; for

  plants also never have any residuum. Across the middle of the body

  of these Ascidians there runs a thin transverse partition, and here it

  is that we may reasonably suppose the part on which life depends to be

  situated.

  The Acalephae, or Sea-nettles, as they are variously called, are not

  Testacea at all, but lie outside the recognized groups. Their

  constitution, like that of the Ascidians, approximates them on one

  side to plants, on the other to animals. For seeing that some of

  them can detach themselves and can fasten upon their food, and that

  they are sensible of objects which come in contact with them, they

  must be considered to have an animal nature. The like conclusion

  follows from their using the asperity of their bodies as a

  protection against their enemies. But, on the other hand, they are

  closely allied to plants, firstly by the imperfection of their

  structure, secondly by their being able to attach themselves to the

  rocks, which they do with great rapidity, and lastly by their having

  no visible residuum notwithstanding that they possess a mouth.

  Very similar again to the Acalephae are the Starfishes. For these

  also fasten on their prey, and suck out its juices, and thus destroy a

  vast number of oysters. At the same time they present a certain

  resemblance to such of the animals we have described as the

  Cephalopoda and Crustacea, inasmuch as they are free and unattached.

  The same may also be said of the Testacea.

  Such, then, is the structure of the parts that minister to nutrition

  and which every animal must possess. But besides these organs it is

  quite plain that in every animal there must be some part or other

  which shall be analogous to what i
n sanguineous animals is the

  presiding seat of sensation. Whether an animal has or has not blood,

  it cannot possibly be without this. In the Cephalopoda this part

  consists of a fluid substance contained in a membrane, through which

  runs the gullet on its way to the stomach. It is attached to the

  body rather towards its dorsal surface, and by some is called the

  mytis. Just such another organ is found also in the Crustacea and

  there too is known by the same name. This part is at once fluid and

  corporeal and, as before said, is traversed by the gullet. For had the

  gullet been placed between the mytis and the dorsal surface of the

  animal, the hardness of the back would have interfered with its due

  dilatation in the act of deglutition. On the outer surface of the

  mytis runs the intestine; and in contact with this latter is placed

  the ink-bag, so that it may be removed as far as possible from the

  mouth and its obnoxious fluid be kept at a distance from the nobler

  and sovereign part. The position of the mytis shows that it

  corresponds to the heart of sanguineous animals; for it occupies the

  self-same place. The same is shown by the sweetness of its fluid,

  which has the character of concocted matter and resembles blood.

  In the Testacea the presiding seat of sensation is in a

  corresponding position, but is less easily made out. It should,

  however, always be looked for in some midway position; namely, in such

  Testacea as are stationary, midway between the part by which food is

  taken in and the channel through which either the excrement or the

  spermatic fluid is voided, and, in those species which are capable

  of locomotion, invariably midway between the right and left sides.

  In Insects this organ, which is the seat of sensation, lies, as

  was stated in the first treatise, between the head and the cavity

  which contains the stomach. In most of them it consists of a single

  part; but in others, for instance in such as have long bodies and

  resemble the Juli (Millipedes), it is made up of several parts, so

  that such insects continue to live after they have been cut in pieces.

  For the aim of nature is to give to each animal only one such dominant

  part; and when she is unable to carry out this intention she causes

  the parts, though potentially many, to work together actually as

  one. This is much more clearly marked in some insects than in others.

  The parts concerned in nutrition are not alike in all insects, but

  show considerable diversity. Thus some have what is called a sting

  in the mouth, which is a kind of compound instrument that combines

  in itself the character of a tongue and of lips. In others that have

  no such instrument in front there is a part inside the mouth that

  answers the same sensory purposes. Immediately after the mouth comes

  the intestine, which is never wanting in any insect. This runs in a

  straight line and without further complication to the vent;

  occasionally, however, it has a spiral coil. There are, moreover, some

  insects in which a stomach succeeds to the mouth, and is itself

  succeeded by a convoluted intestine, so that the larger and more

  voracious insects may be enabled to take in a more abundant supply

  of food. More curious than any are the Cicadae. For here the mouth and

  the tongue are united so as to form a single part, through which, as

  through a root, the insect sucks up the fluids on which it lives.

  Insects are always small eaters, not so much because of their

  diminutive size as because of their cold temperament. For it is heat

  which requires sustenance; just as it is heat which speedily

  concocts it. But cold requires no sustenance. In no insects is this so

  conspicuous as in these Cicadae. For they find enough to live on in

  the moisture which is deposited from the air. So also do the

  Ephemera that are found about the Black sea. But while these latter

  only live for a single day, the Cicadae subsist on such food for

  several days, though still not many.

  We have now done with the internal parts of animals, and must

  therefore return to the consideration of the external parts which have

  not yet been described. It will be better to change our order of

  exposition and begin with the animals we have just been describing, so

  that proceeding from these, which require less discussion, our account

  may have more time to spend on the perfect kinds of animals, those

  namely that have blood.

  6

  We will begin with Insects. These animals, though they present no

  great multiplicity of parts, are not without diversities when compared

  with each other. They are all manyfooted; the object of this being

  to compensate their natural slowness and frigidity, and give greater

  activity to their motions. Accordingly we find that those which, as

  the (Millipedes), have long bodies, and are therefore the most

  liable to refrigeration, have also the greatest number of feet. Again,

  the body in these animals is insected-the reason for this being that

  they have not got one vital centre but many-and the number of their

  feet corresponds to that of the insections.

  Should the feet fall short of this, their deficiency is

  compensated by the power of flight. Of such flying insects some live a

  wandering life, and are forced to make long expeditions in search of

  food. These have a body of light weight, and four feathers, two on

  either side, to support it. Such are bees and the insects akin to

  them. When, however, such insects are of very small bulk, their

  feathers are reduced to two, as is the case with flies. Insects with

  heavy bodies and of stationary habits, though not polypterous in the

  same way as bees, yet have sheaths to their feathers to maintain their

  efficiency. Such are the Melolonthae and the like. For their

  stationary habits expose their feathers to much greater risks than are

  run by those of insects that are more constantly in flight, and on

  this account they are provided with this protecting shield. The

  feather of an insect has neither barbs nor shaft. For, though it is

  called a feather, it is no feather at all, but merely a skin-like

  membrane that, owing to its dryness, necessarily becomes detached from

  the surface of the body, as the fleshy substance grows cold.

  These animals then have their bodies insected, not only for the

  reasons already assigned, but also to enable them to curl round in

  such a manner as may protect them from injury; for such insects as

  have long bodies can roll themselves up, which would be impossible

  were it not for the insections; and those that cannot do this can

  yet draw their segments up into the insected spaces, and so increase

  the hardness of their bodies. This can be felt quite plainly by

  putting the finger on one of the insects, for instance, known as

  Canthari. The touch frightens the insect, and it remains motionless,

  while its body becomes hard. The division of the body into segments is

  also a necessary result of there being several supreme organs in place

  of one; and this again is a part of the essential constitution of

  inse
cts, and is a character which approximates them to plants. For

  as plants, though cut into pieces, can still live, so also can

  insects. There is, however, this difference between the two cases,

  that the portions of the divided insect live only for a limited

  time, whereas the portions of the plant live on and attain the perfect

  form of the whole, so that from one single plant you may obtain two or

  more.

  Some insects are also provided with another means of protection

  against their enemies, namely a sting. In some this is in front,

  connected with the tongue, in others behind at the posterior end.

  For just as the organ of smell in elephants answers several uses,

  serving alike as a weapon and for purposes of nutrition, so does

  also the sting, when placed in connexion with the tongue, as in some

  insects, answer more than one end. For it is the instrument through

  which they derive their sensations of food, as well as that with which

  they suck it up and bring it to the mouth. Such of these insects as

  have no anterior sting are provided with teeth, which serve in some of

  them for biting the food, and in others for its prehension and

  conveyance to the mouth. Such are their uses, for instance, in ants

  and all the various kinds of bees. As for the insects that have a

  sting behind, this weapon is given them because they are of a fierce

  disposition. In some of them the sting is lodged inside the body, in

  bees, for example, and wasps. For these insects are made for flight,

  and were their sting external and of delicate make it would soon get

  spoiled; and if, on the other hand, it were of thicker build, as in

  scorpions, its weight would be an incumbrance. As for scorpions that

  live on the ground and have a tail, their sting must be set upon this,

  as otherwise it would be of no use as a weapon. Dipterous insects

  never have a posterior sting. For the very reason of their being

  dipterous is that they are small and weak, and therefore require no

  more than two feathers to support their light weight; and the same

  reason which reduces their feathers to two causes their sting to be in

  front; for their strength is not sufficient to allow them to strike