122 
ANIMAL PEDIGREES. 
June, 1891. 
flounder, the left eye is completely twisted over to the 
right side of the head, and lies immediately above the true 
right eye, which is displaced downwards, and lies immedi¬ 
ately behind the angle of the mouth. 
Crabs differ markedly from their allies, the lobsters, in 
the small size and rudimentary condition of their abdomen 
or “ tail.” Development, however, affords abundant evidence 
of the descent of crabs from macrourous ancestors. A crab 
leaves the egg in what is termed the zoea condition, Fig. 8, 
possessing a long and clearly-jointed abdomen ; and through¬ 
out all the earlier stages of existence the abdomen remains 
at least as long as the body. At the megalopa stage, repre¬ 
sented in Fig. 9, the shape and proportions are very similar 
to those of a lobster or other macrourous decapod. It is only 
in the last stages of development when the shape, though 
not the size, of the adult crab is attained, that the abdomen 
becomes relatively smaller, and is turned forwards out of 
sight beneath the hinder part of the thorax. 
Molluscs afford excellent illustrations of recapitulation. 
The typical gastropod has a large spirally-coiled shell; the 
limpet, however, has a large conical shell, which in the 
adult gives no sign of spiral twisting, although the structure 
of the animal shows clearly its affinity to forms with spiral 
shells. Development solves the riddle at once, telling us 
that in its early stages the limpet embryo has a spiral shell, 
which is lost on the formation, subsequently, of the conical 
shell of the adult. 
Recapitulation is not confined to the higher groups of 
animals, and the Protozoa themselves yield most instructive 
examples. A very striking case is that of Orbitolites, one of 
the most complex of the porcellaneous Foraminifera, in which 
each individual during its own growth and development passes 
through the series of stages by which the cyclical or discoidal 
type of shell was derived from the simpler spiral form. 
The fully-formed Orbitolite shell is a thin calcareous 
disc shown full face in Fig. 4 and in profile in Fig. 8. It is 
hollow, and the central cavity is divided into chambers by 
concentric partitions or septa, well seen in Fig. 4. These 
chambers are further subdivided by incomplete radial par¬ 
titions. The concentric partitions are perforated by numer¬ 
ous holes, which place the chambers in communication with 
one another; and the outermost or marginal chamber 
communicates with the outer world through a series of 
holes round the edge or rim of the disc, seen in Fig. 8. 
During life all the cavities are filled with a slimy protoplasm, 
very similar to that of which an Amoeba consists; through 
the perforations in the septa the protoplasm of one chamber 
