150 
ANIMAL PEDIGREES. 
July, 1891. 
berger has specially noticed this tendency in Ammonites. 
Many early larvae show it markedly, the explanation in this 
case being that it is essential for them to hatch in a condition 
capable of independent existence, i.e ., capable, at any rate, of 
obtaining and digesting their own food. 
Anachronisms, or actual reversal of the historical order 
of development of organs or parts, occur frequently. Thus 
the joint surfaces of bones acquire their characteristic curva¬ 
tures before movement of one part on another is effected, and 
before even the joint cavities are formed. 
Another good example is afforded by the development of 
the mesenterial filaments in Alcyonarians. Wilson has shown 
in the case of Renilla that in the development of an embryo 
from the egg the six endodermal filaments appear first, and 
the two long ectodermal filaments at a later period; but that 
in the formation of a bud this order of development is 
reversed, the ectodermal filaments being the first formed. 
He suggests, in explanation, that as the endodermal filaments 
are the digestive organs, it is of primary importance to the 
free embryo that they should be formed quickly. The long 
ectodermal filaments are chiefly concerned with maintaining 
currents of water through the colony; in bud-development 
they appear before the endodermal filaments, because they 
enable the bud during its early stages to draw nutrient 
matter from the body fluid of the parent; while the endo¬ 
dermal filaments cannot come into use until the bud has 
acquired its own mouth and tentacles. 
The completion of the ventricular septum in the heart 
of higher vertebrates before the auricular septum is an often 
quoted anachronism, and every embryologist could readily 
furnish many other cases. 
A curious instance is afforded by the development of the 
teeth in mammals, if recent suggestions as to the origin of the 
milk dentition are confirmed, and the milk dentition prove to 
be a more recent acquisition than the permanent one. 
Distortion of a curious kind is seen in cases of abrupt 
metamorphosis, where, as in the case of many Eehinoderms, 
of Phoronis, and of the metabolic insects, the larva and the 
adult differ greatly in form, habits, mode of life, and very 
usually in the nature of their food and the mode of obtaining 
it; and the transition from the one stage to the other is not 
a gradual but an abrupt one, at any rate so far as external 
characters are concerned. 
Sudden changes of this kind, as from the free swimming 
Pluteus to the creeping Echinus, or from the sluggish leaf- 
eating caterpillar to the dainty butterfly, cannot possibly be 
