Biology Reference
In-Depth Information
Fig. 4.23
the fact that the bottom of the caecum is a dead-end - it is a side pouch in the
digestive tract. The caecum is essential for the digestion of plant material because it
contains bacteria that can break down cellulose, which animal cells cannot. Because
the caecum is a dead-end sac, it allows a permanent colony of cellulose-digesting
bacteria more time to digest tough plant material. The size of the caecum in dif-
ferent animals reflects the proportion of the diet that is derived from plants (see
Fig. 4.23).
Modern humans are omnivorous and so lack a caecum, but in its place is a
smaller organ called the appendix, which is essentially the remnant of the apex of
the caecum that our ancestors possessed. The appendix is not essential for life and
indeed may need to be surgically removed if it becomes infected. It may have some
functional roles, such as providing a reservoir of bacteria that aid digestion.
On the right of Fig. 4.23 is shown an example of vestigial pelvic girdles in whales
and the occurrence of a tail in early human embryos. All the whales and dolphins
have vestigial pelvic girdles, and many have, in addition, the remnants of femurs.
These structures no longer function in locomotion, and are not attached to the ver-
tebral column, as they are in fossil ancestors of whales. All mammals have a tail
at some point in their development, whether or not the adult animals have tails. In
humans, the tail is present during the first one to two months after conception, and is
then destroyed by a destruction pathway called programmed cell death. Figure 4.23
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