ANIMAL SOCIETIES — SNODGRASS 427 



THE COELENTERATA 



The coelenterates are many-celled aquatic animals, mostly marine, 

 that occur in two principal forms, one typified by the hydras and 

 sea anemones, the other by the jellyfish, or medusae. The two, how- 

 ever, are merely versions of one type of structure, the polyp. The 

 simplest example of the polyp form is the fresh- water Hydra^ which 

 is a small, double-walled cylindrical bag attached by one end to a 

 support, open at the other by the mouth, which is surrounded by a 

 circle of tentacles. The inner cavity is the stomach. The jellyfish 

 is a polyp flattened lengthwise into the form of a bowl or umbrella, 

 which floats upside down in the water. The mouth is on a central 

 projection of the undersurf ace ; the tentacles hang from the margin. 



Hydra propagates itself by lateral branches that assume the parent 

 structure and then break off as new hydras. Other related forms also 

 produce new individuals by branching, but they remain attached to 

 the parent stem. There results therefore a branched, plantlike colony 

 of polyps, which never occur as free individuals. A well-known exam- 

 ple of tliis type of colonial coelenterate is the genus Ohella, It grows 

 in the ocean as masses of fine branching threads attached by root 

 processes to shells, stones, and piers along the shore. Most of the 

 polyps are feeding members of the colony ; their stomachs are all con- 

 nected through the branches and the main stem. We have here there- 

 fore a colony of individuals interdependent by the possession of a 

 common stomach. 



From the lower part of the Ohelia stem are produced branches that 

 do not take the form of polyps. They are simple shafts with enlarged 

 ends, each enclosed in a transparent sheath. Along the shaft are 

 formed buds that develop into small medusae, which produce the 

 sexual elements; these unite as eggs and speimatozoa and give rise 

 to simple larvae known as planulae. The larvae become attached at 

 one end to a support and develop into polyps that branch and form 

 new Ohella colonies. The Ohelia colony therefore produces both 

 polypoid and medusoid individuals, and the life of the species alter- 

 nates between a fixed nonreproductive form, and a free reproductive 

 form that produces the sex elements. In the genus Bougainmllea 

 the medusae are produced as individual buds from the stems as are 

 the polyps. The common "jellyfish" are medusae that have dispensed 

 with the colonial polyp stage. 



There are various other members of the Coelenterata that form 

 branched colonies. When the stems and branches produce a cal- 

 careous skeleton, the colony is known as a coral. Corals, however, oc- 

 cur in at least five coelenterate groups. There may be thousands or 

 perhaps millions of polyps in a single coral colony. The so-called 

 stony corals, or madrepores, are the builders of coral reefs and coral 



