902 ^^^ ANIMAL KINGDOM 



The ability to correct disarrangements is usually not as marked as 

 the regenerative ability. If an oral end ol one hydra is grafted onto the 

 side of another, the animal will eventually divide to form two normal 

 individuals. If, however, the cut surfaces of two oral ends are placed 

 together, they heal to form a single individual with two mouths and no 

 base. Such monsters remain thus, apparently unable to achieve the 

 normal form. If such a creature is then cut in half, each half may re- 

 generate a base. 



In the ctenophores, pieces put together in the original orientation 

 with oral and aboral ends aligned will usually regulate into normal 

 individuals, while opposed pieces such as two aboral ends with their 

 cut surfaces placed together will not. 



A most remarkable example of successful rearrangement is found 

 in hydra. As in most polyps the mouth can open very wide, and it is 

 possible to turn a hydra inside out through its mouth without tearing 

 any of the tissues! Such an individual, with the epidermis inside and 

 gastrodermis outside, is unable to turn inside out again to recover its 

 normal form. It does regain its normal form, however, by a direct mi- 

 gration of the individual cells across the thin mesogleal layer to their 

 former location. 



These abilities to repair or replace parts and to rearrange dis- 

 arrangements are two aspects of form regulation, the processes by which 

 individual organisms come to have the morphology of their kind. Most 

 coelenterates are able to regulate throughout their life. Injuries or 

 disarrangements are as easily corrected by embryos as by adults. Among 

 the ctenophores, however, the embryo has much less regulative ability 

 than the adult. If the two-cell stage is divided into two separate cells, 

 each becomes only half a ctenophore. One cell from the four-cell stage 

 becomes one quarter of a ctenophore having only two comb columns. 

 Later in life, however, those that survive will spontaneously regenerate 

 the missing parts to become normal. 



Not all coelenterates have good regulative abilities. The siphono- 

 phores, for example, usually fail to replace lost parts, and wounds are 

 healed by a simple closure of the hole. In this group regulative ability 

 is good in the embryos and larvae and becomes poor in the adults. 



Many other animal groups, including the sponges already discussed 

 and such complex animals as crabs, starfishes and salamanders, have a 

 considerable ability to regulate form. The coelenterates and ctenophores 

 are especially suitable for experimentation because the body plan, while 

 relatively simple, is geometrically exact and provides an excellent frame 

 of reference. Survival after operations is not difficult to achieve. The 

 phenomena associated with form regulation are considered to be similar 

 to those of embryologic development. 



Questions 



1. Define "planktonic" and "plankton." 



2. Describe mesoglea. 



3. Draw a vertical section through a jellyfish and a sea anemone. 



