26 STRUCTURE AND FUNCTION. 
or sac which is then termed the cwlom. This ccelom does 
not usually fill the whole primary body-cavity for a large 
part of the latter remains as the Aemocele or blood-space. 
In the Calomata the typical excretory organs are excretory 
tubules or xephridia which open directly into the cavity of 
the coelom. The comparative size of the coelom and hzemo- 
ceele varies greatly. 
The actual connection, if any, between the two tridermic types is 
not known. The archiccelic is evidently the simplest, but it is doubt- 
ful whether in the evolution of the Ca/omata, the flame-cell tubules 
become transformed into nephridia, whether they were merely replaced 
by the latter in function and atrophied, or whether the ancestors of 
Celomata never had flame-cell tubules. 
In a general way, these three types correspond to the 
three forms of symmetry; the monodermic organism is 
centro-symmetric, the didermic usually axo-symmetric, and 
the tridermic is in nearly all cases plano-symmetric. 
Structure and Function.—Organs of the body are 
of certain form and structure according to the functions 
they perform. Hence there is a general similarity in the 
form of the different systems referred to in the last chapter. 
Nervous systems, for example, have certain striking resem- 
blances throughout the whole animal kingdom, and so with 
all other primary systems. 
We can only notice here two important parts of this sub- 
ject. Firstly, theré are many instances of loss of function. 
This invariably leads to reduction or complete extinction 
of the organ in question. The most endoparasitic animals, 
such as tape-worms, undergo a complete loss of all ali- 
mentary organs as they are not required. Again, in many 
cases the organs persist as mere vestiges and are then 
known as vestigial organs. Remarkable instances of these 
are the hind limbs of whales, some of the jaws of the cray- 
fish, and the splint-bones of the horse. 
Other organs are just acquiring the function which is 
raising them into importance and are still small. These 
may be called rudimentary organs. Organs like everything 
else in the world, have their rise, their culminating point, and 
their fall. A ves¢¢géaZ organ is in the last phase of its history, 
whilst a vzd/mentary organ is in the first. The electric organ 
