170 
ANIMAL PEDIGREES. 
Aug.. 1891. 
antennules in front and six pairs of swimming legs behind. 
The Nauplius eye is still present, a little way behind the basal 
joints of the antennules. 
Now comes the great change. The pupa, meeting with 
a crab, fastens itself to the under surface of the crab’s tail by its 
antennules and then goes to the bad with startling rapidity. 
Within three hours of the time of fixing itself to the crab, the 
six pairs of swimming legs, with the muscles moving them, 
and the whole posterior part of the body rot away, and are 
cast off as a slough. The antennules become modified into a 
tube, piercing the skin of the crab ; the head of the Sacculina 
remains as a bottle-shaped mass in connection with the 
modified antennules, but the bivalved carapace, with all the 
other organs, including the eye, are cast off and lost, Fig. 13. 
The Sacculina now passes for a time completely into the 
interior of the crab : later on, after increasing in size, it comes 
once more to the surface and becomes the bag-like mass which 
we have found to be the adult condition, Fig, 14. 
This is a typical instance of degeneration or retrograde 
development, the animal being more highly organised, and 
standing at a higher morphological level in its early stages 
than when adult. Yet, inasmuch as the organs that are lost, 
such as the limbs and eve, would be of no use to it in its 
changed conditions of life, there is nothing in the whole history 
that is in any way inconsistent with natural selection. 
This principle of degeneration, recognised by Darwin as a 
possible, and, under certain conditions, a necessary conse¬ 
quence of his theory, has been since advocated strongly by 
Dohrn, and later by Lankester, in an evening discourse 
delivered before the British Association at the Sheffield 
meeting, in 1879. Both Dohrn and Lankester have suggested 
that degeneration may occur much more widely than is 
commonly supposed. 
In animals which are parasitic when adult, but free 
swimming in their early stages, as in the case of Sacculina, 
degeneration is clear enough; so also is it in the case of the 
solitary Ascidians, in which the larva is a free swimming 
animal with a notochord, an elongated tubular nervous 
system, and sense organs, while the adult is fixed, devoid of 
the swimming tail, with no notochord, and with a greatly 
reduced nervous system and aborted sense organs. 
In such cases the animal, when adult, is, as regards the 
totality of its organisation, at a distinctly lower morphological 
level, is less highly differentiated than it is when young, and 
during individual development there is actual retrograde 
development of important systems and organs, 
