Chap. 23 COELENTERATES — SIMPLE MULTICELLULAR ANIMALS 481 



The medusa's umbrella-shaped body is largely the jellied mesoglea contain- 

 ing at least 95 per cent water and the scattered fibers and cells that have 

 migrated into it. Both the upper convex and under concave surfaces are cov- 

 ered by epidermis liberally-supplied with stinging cells and sensory nerve cells. 

 The mouth is at the end of a short tube (manubrium) which hangs from the 

 center of the under surface (Fig. 23.10). The passage from it opens into a 

 central cavity from which four radial canals lead to the circular canal that 

 extends around the margin of the umbrella. All of these canals are parts of the 

 enteron which in its evolution has added the distribution of food to its already 

 established functions of digestion and absorption. As might be expected of an 

 active free-living animal, the nerve cells and their associations in the medusa 

 are much more highly developed than in the attached polyps. A nerve ring 

 lies along the margin of the umbrella. This receives processes of nerve cells 

 acting in a simplified way like the central nervous system of higher animals. 

 A sensory organ of equilibrium (statocyst) is located at the base of every 

 other tentacle. These and similar sensory organs in other medusae are consid- 

 ered as the first real organs to be developed in the invertebrates. 



Reproduction, The medusae of any one colony are either all male or all 

 female. They closely resemble one another and the ovaries and testes develop 

 in the same relative position beneath the radial canals. The sex cells are shed 

 into the open water where fertilization occurs. The embryo becomes first a 

 spherical blastula, then a swimming larva. Its wanderings are important to the 

 distribution of the species but they last only a few hours before it settles upon 

 rock or seaweed and the development of the colony of polyps begins (Fig. 

 23.10). 



Gonionemus — A Hydrozoan Jellyfish. In Gonionemus, the medusa is well 

 developed and the polyp is diminutive. The medusa is as transparent as glass 

 and less than an inch in diameter (Fig. 23.12). This was formerly a common 

 jellyfish among the eel grasses along the eastern coast of the United States. 



When feeding, Gonionemus swims toward the surface with its mouth down. 

 There it turns over and floats slowly downward, its mouth up and its tentacles 

 extended in a wide open snare for any small animals within reach of their 

 stinging clutch. When at rest it likewise lies mouth up, with its tentacles at- 

 tached to the bottom by the adhesive pads. 



Other Species of Hydrozoa. The skeletons of hydrocorallines are peppered 

 with minute pores but the polyps are seldom seen extending from them since 

 they expand at night. The "stinging coral" (Millepora alicornia) well known 

 along the coast of Florida contributes largely to the formation of coral reefs. 



The colonies of Hydractinia which live on the shells of hermit crabs have a 

 division of labor similar to that of Obelia but in these colonies there are 

 feeding polyps, reproductive polyps with medusa buds, and protective polyps 

 without mouths, only stubby tentacles and a great supply of stinging cells. 



