8 Invertebrata. 



Morphology. Groups of organs are generally sym- 

 metrically disposed in animals ; either they are arranged 

 in order around a central point in one plane, or else each 

 individual consists of a succession of similar segments, 

 as in a centipede. In the first case the symmetry is 

 said to be radial as in the star-fish (fig. 3) ; in the 

 second the segments are each made up of two sym- 

 metrical halves, and the symmetry is said to be 

 bilateral 



In a perfectly symmetrical animal all the organs 

 should be proportionally developed, but as the vary- 

 ing conditions of animal existence often require the 

 more extensive performance of some duties than of 

 others, we always find that some organs are larger, 

 others smaller. In fact animals are so perfectly fitted 

 to their surroundings that could we know all the con- 

 ditions under which a given animal existed, we could 

 form a good conception of its structure and vice versd. 



The Embryo. To understand the true relations of 

 structures in animals it is necessary to watch the growth 

 of the organism from the earliest stages of its produc- 

 tion in the egg until it attains its adult condition. The 

 embryo is not a simple miniature of the full grown 

 animal, but reaches its perfect state by undergoing a 

 series of changes, which follow each other in a definite 

 order. In this process, parts and organs start into being 

 which were before unnoticeable, and some of these have 

 only a transient existence fading off into nothingness. 

 Thus the common acorn shell emerges from its egg as a 

 little free-swimming larva (fig. 4), with eyes and feelers, 

 but these totally vanish in the adult (fig. 5) ; such organs 

 are known as provisional organs, and occasionally they 



