nnulose animals),
Testacea (= molluscs, echinoderms, ascidians). A class of Acalephae,
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, mammae, 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 mammae. 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 natu