88 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



conceived of as alternating with the hydroid colony, or asexual generation. 

 It would probably be advisable to drop this idea altogether and to regard the 

 medusa simply as the final or adult stage in the development of the organism. 



C. GENERAL SURVEY OF OTHER COELENTERATES 



Study the collection of coelenterates provided for the class, reading what 

 Hegner has to say about them (pp. 139-43). Acquaint yourself with the general 

 appearance of the following chief groups of coelenterates: 



1. The hydroids. These animals are extremely common along the shores 

 of the ocean, forming beautiful plantlike growths on rocks, wharves, plants, 

 shells of animals, etc. Note various types of branching, size and shape of 

 zooids, etc. 



2. Medusae. Medusae, as stated above, are the sexual stage of the hydroid 

 colonies. They are small, gelatinous, bell-shaped animals with tentacles hanging 

 free from the edge of the bell and a mouth dependent from the center of the 

 concave surface of the bell. From the mouth, canals (usually four) radiate to 

 the periphery of the bell and serve as a food-distributing system. A circular 

 muscular shelf, the velum, extends inward from the edge of the bell; its contrac- 

 tions enable the animal to swim. The jelly-like composition of the medusae is 

 due to the enormous development of the mesogloea. 



3. True jellyfishes. These differ from the medusae, which are also often 

 called jellyfishes, through the absence of a velum, larger size and more saucer- 

 like form, and more complicated structure. They usually do not pass through 

 a hydroid stage. 



4. Siphonophora. These strange free-floating hydroid colonies illustrate the 

 principle of division of labor carried to its highest development, since they may 

 consist of seven or eight different types of individuals. The famous Physalia, 

 or Portuguese man-of-war has a large "float" (which is a modified medusa), from 

 the lower side of which the other members of the colony hang down, with long 

 trailing tentacles. The Velella, or purple sail, has a flattened disklike float 

 bearing an erect "sail," with zooids on the under surface of the float. In other 

 types of Siphonophora the float is lacking but the upper part of the colony 

 consists of a circle or a long chain of swimming bells, which are modified medusae. 



5. Sea anemones. These beautiful animals of the seashore are similar in 

 appearance to hydroid polyps, but their internal structure is somewhat more 

 complicated and they have no medusa stage. They have a stout column with 

 an oral disk covered with tentacles. 



6. Stony corals. The common corals which build up the keys and reefs are 

 small animals of the same structure as sea anemones. They secrete an external 

 skeleton of calcium carbonate, which persists after they perish. In all dry speci- 

 mens of coral the animals are of course destroyed, but the cuplike depressions 



