40 The Development of o Primitive Animal 



The roots and stems of the colony are hollow cylinders composed of 

 the same two tissue layers, epidermis and gastrodermis. The internal 

 space, called the coelenteron, is filled with fluid, is continuous, and extends 

 into the polyps. This means that the feeding and reproductive polyps and 

 the cells in the walls of runners and stems are all in intimate contact, and 

 all parts of the colony can be supplied with the food materials that enter 

 through the feeding polyps. 



Both kinds of polyp are initiated as buds on the stems. At first the 

 bud is simply a protuberance of the stem wall that contains both tissue 

 layers. The protuberance lengthens rapidly by cell division and by migra- 

 tion from other areas. Infoldings and constrictions of the tissue ultimately 

 give rise to the organs that are characteristic of the feeding and reproduc- 

 tive polyps. 



As we have mentioned, the medusae also are formed by budding 

 from the central cylinder of the reproductive polyps. Fully formed, they 

 are minute jellyfish, shaped like an umbrella and rimmed with tentacles. 

 The mouth hangs down from the concave side and leads upward into the 

 digestive cavity in the middle of the umbrella. They move by jet propul- 

 sion, water being forced out of the mouth, and feed upon Crustacea and 

 protozoa. They are of two sexes, male and female. Their gonads develop 

 inside the digestive cavity, and eggs or sperm are propelled through the 

 mouth. Fertilization takes place in the sea. 



The fertihzed egg promptly cleaves into two, the two into four, and 

 so on until a hollow ball of cells is produced. This is called the blastula 

 stage and corresponds to a similar stage in the development of vertebrate 

 embryos ( see Chapter 5 ) . Some of the cells are forced into the interior of 

 the ball and become the gastrodermis, while the cells that remain outside 

 become the epidermis. This corresponds to the gastrula stage of embryo- 

 genesis, encountered in higher invertebrates and vertebrates, during 

 which their three basic tissue layers appear. After tissue separation, the 

 Obelia embryo elongates, and cilia appear over the outer surface. The 

 constituent cells now begin to transform into sensory, gland, and muscle 

 cells and cnidoblasts. At this point, embryogenesis is concluded, and the 

 animal is called a planula larva (i.e., the immature, larval form of the 

 adult polyp ) . The planula swims about for a period of hours to days and 

 then attaches to a rock or shell. The attached end develops into a root- 

 like runner, and the free end acquires a mouth and tentacles. Buds appear, 

 the polyp colony is initiated, and the cycle is complete. 



Polymorphism in the Coelenterotes 



Polymorphism (a variety of forms) occurs at two levels in the coe- 

 lenterates. First, there is the basic difference in the adult structures (i.e., 



