Form and Function | Page 4

Among the "exsanguineous" animals, however,
corresponding to our Invertebrates, he established a much more definite
classification than the popular, which is apt to call them
indiscriminately "shellfish," "insects," or "creeping things." He went
beyond the superficialities of popular classification, too, in clearly
separating Cetacea from fishes. He had some notion of species and
genera in our sense. He distinguished many species of
cuttlefish--Octopus (Polypus) of which there were many kinds, Eledone
(Moschites) which he knew to have only one row of suckers while
Octopus has two, Argonauta, Nautilus, Sepia, and apparently Loligo
media (= his Teuthis) and L. vulgaris(or forbesii) which seems to be his
Teuthos. He had a grasp of the principles which should be followed in
judging of the natural affinities of species. For example, he knew that
the cuckoo resembles a hawk. "But," he says, "the hawk has crooked
talons, which the cuckoo has not, nor does it resemble the hawk in the
form of its head, but in these respects is more like the pigeon than the
hawk, which it resembles in nothing but its colour; the markings,
however, upon the hawk are like lines, while the cuckoo is spotted"
(Hist. Anim., Cresswell's trans., p. 147, London, 1862).
The groups he distinguished were--man, viviparous quadrupeds,
oviparous quadrupeds, birds, fishes, Cetacea, Cephalopoda,

Malacostraca (= higher Crustacea), Insecta (= annulose animals),
Testacea (= molluscs, echinoderms, ascidians). A class of Acalephæ,
including sea-anemones and sponges, was grouped with the Testacea.
The first five groups were classed together as sanguineous, the others
as exsanguineous, from the presence or absence of red blood.
Besides these classes "there are," he says, "many other creatures in the
sea which it is not possible to arrange in any class from their scarcity"
(Creswell, loc. cit., p. 90).
(3) Aristotle's greatest service to morphology is his clear recognition of
the unity of plan holding throughout each of the great groups.
He recognises this most clearly in the case of man and the viviparous
quadrupeds, with whose structure he was best acquainted. In the
Historia Animalium he takes man as a standard, and describes his
external and internal parts in detail, then considers viviparous
quadrupeds and compares them with man. "Whatever parts a man has
before, a quadruped has beneath; those that are behind in man form the
quadruped's back" (Cresswell, loc. cit., p. 26). Apes, monkeys, and
Cynocephali combine the characteristics of man and quadrupeds. He
notices that all viviparous quadrupeds have hair. Oviparous quadrupeds
resemble the viviparous, but they lack some organs, such as ears with
an external pinna, mammæ, hair. Oviparous bipeds, or birds, also "have
many parts like the animals described above." He does not, however,
seem to realise that a bird's wings are the equivalent of a mammal's
arms or fore-legs. Fishes are much more divergent; they possess no
neck, nor limbs, nor testicles (meaning a solid ovoid body such as the
testis in mammals), nor mammæ. Instead of hair they have scales.
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