THE SPONGES AND THE TWO-LAYERED ANIMALS 



151 



medusa buds. As the buds mature they ap- 

 proach the distal end of the blastostyle, 

 finally leaving the gonotheca through a 

 terminal opening and swimming away. 

 These free-living medusae are the familiar 

 jellyfish, which, since they are able to move 

 about, spread the species to new areas. 



Medusae develop gonads which produce 

 eggs and sperms that unite in the sea water. 

 The resulting embryo, the planula, swims 

 about for a time but eventually settles to 

 the bottom, becomes attached to the sub- 

 stratum, and grows into an asexual colony, 

 the polyp thus completing the cycle. This 

 species is an excellent illustration of meta- 

 genesis. 



The medusa of obelia appears to be quite 

 different from the polyp, but basically they 

 resemble each other closely. The former has 

 a central hanging manubrium, located on 

 the concave side. At the center of the manu- 

 brium is the mouth, which opens into four 

 radial canals, continuing into the circular 

 ring canal in the margin of the bell. This 

 constitutes the coelenteron and is equivalent 

 to the same organ in hydra or the polyp of 

 obelia. The space between the epidermis 

 and the coelenteron is filled with the rather 

 extensive gelatinous mesogloea. The medusa 

 of obelia is microscopic and these struc- 

 tures can best be seen in larger jellyfish 

 such as Gonionemiis, which is commonly 

 used for laboratory studies (Fig. 8-14). 



Obelia illustrates the beginning of divi- 

 sion of labor among the polyps, but this 

 is carried to a greater degree of efficiency 

 among some of the other Hydrozoa. For 

 example, in Hydractinia (Fig. 8-22), a 

 colonial form that is often found on the 

 shells of hermit crabs, some of the polyps 

 gather food, others reproduce the species, 

 and still others protect the colony by use 

 of large batteries of nematocysts. This is 

 one step higher than obelia. Polymorphism 

 (many shapes), the name applied to this 

 type of division of labor, is carried even 

 farther in Phijsalia, the Portuguese man-of- 

 war (Fig. 8-15), which comprises at least 



four types of individuals. In addition to the 

 various types of individuals found in the 

 Hydractinia colony, Physalia also has a type 

 that forms a gas-filled float which supports 

 the colony on the surface of the sea as it 

 is borne here and there by the wind and 

 currents in a never-ending search for food. 

 The tentacles bear unusually large nemato- 

 cysts which are occasionally a menace to 

 bathers if they happen to become entangled 

 in them. This can easily happen in some 

 species of Physalia, since the tentacles trail 

 as much as 50 feet beneath and behind the 

 float. Physalia has little difficulty in paralyz- 

 ing a fish several inches in length and even- 

 tually consuming it. The many types of 

 individuals in this colony illustrate poly- 

 morphism in its most advanced form among 

 the coelenterates. 



Other coelenterates 



The second class of coelenterates, the 

 Scyphozoa, includes most of the larger 

 jellyfishes (Fig. 8-16) which either have a 

 reduced polyp stage or lack it altogether. 

 Aurelia (Fig. 8-22), one of the most com- 

 mon examples, is found in great numbers 

 up and down the coasts of the United 

 States. Its rather flattened umbrella is 

 fringed with small tentacles that are inter- 

 rupted at eight equally spaced spots where 

 a sense organ is located. The rhythmic con- 

 tractions of the circular muscle in the bell 

 are responsible for the graceful movement 

 of this beautiful creature. Four long oral 

 lobes, which are located on the short manu- 

 brium, are heavily armed with nematocysts 

 and function like tentacles in capturing 

 food and directing it into the mouth. The 

 coelenteron is divided into four large gastric 

 pouches from which radiate smaller canals 

 that connect with the ring canal at the 

 periphery of the bell. The gonads, lying in 

 the gastric pouches, form four horseshoe- 

 shaped bodies when viewed from the 

 aboral (opposite the mouth) side, and con- 

 stitute a ready mark of identification. 



The sense organs, consisting of eye-spots 



