26 STRUCTURE AND FUNCTION, 



or sac which is then termed the coelom. This coelom does 

 not usually fill the whole primary body-cavity for a large 

 part of the latter remains as the hcemoccele or blood-space. 

 In the Cci'lomata the typical excretory organs are excretory 

 tubules or nephridia which open directly into the cavity of 

 the coelom. The comparative size of the coelom and hsemo- 

 coele varies greatly. 



The actual connection, if any, between the two tiidermic types is 

 not known. The archicoelic is evidently the simplest, but it is doubt- 

 ful whether in the evolution of the Ccelomata, 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 

 Coclotnata 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 piano-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, there 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 tbeni 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 vestigial oxgzxi is in the last phase of its history, 

 whilst a rudiinentary organ is in the first. The electric organ 



