185 J""^ ANIMAL KINGDOM 



(sometimes only those holding the prey) and bends them toward the mid- 

 dle ot the sLibumbrellar surface. The side of the umbrella holding the 

 prey shrinks and bends inward. This movement brings the prey toward 

 the mouth, an opening at the end of a short tube, the manubrium, that 

 hangs down from the middle of the subumbrellar surface. As tfie prey 

 is brought toward the mouth the manubrium extends and bends toward 

 the prey. This synchronized activity involves the longitudinal muscle 

 fibers of the tentacles, radial muscle fibers beneath the subumbrellar epi- 

 dermis, and both circular and longitudinal fibers in the manubrium. 

 Swimming muscles are not involved. 



Surrounding the mouth are four lips, each folded longitudinally. 

 The surface on the inner side of the fold, toward the mouth, is ciliated. 

 Mucus is secreted on this surface and the ciliary activity moves the 

 mucous sheet steadily into the mouth. As soon as tfie lips, which are 

 weakly muscular, have folded over the prey, the tentacles release the 

 bases of the discharged nematocysts and the medusa resumes its nor- 

 mal shape. 



Digestion. The mouth opens into a large stomach in the middle 

 of the medusa. When the prey has been swallowed, the mouth closes 

 tightly and some of the gastrodermis cells secrete a digestive juice con- 

 taining proteases. These enzymes initiate the breakdown of protein and 

 reduce the prey to a broth. 



Close to the subumbrellar surface the stomach extends laterally as 

 four radial canals (Fig. 10.2), which are continuous at the margin with 

 a circular canal, from which small branches extend into the tentacles. 

 Both the stomach and the canals are lined with tracts of ciliated gastro- 

 dermis cells which set up currents to circulate the broth throughout the 

 system. Other gastrodermis cells absorb dissolved nutrients and ingest 

 the remaining small food particles. Ingestion is the same as in many 

 protozoans. Particles are taken up in food vacuoles where digestion of 

 fats and carbohydrates, and further digestion of proteins, take place. As 

 among protozoans, the vacuoles become acid and then alkaline during 

 digestion. 



Since the stomach and canals perform both circulatory and digestive 

 functions, digesting the food and distributing it to all parts of the body, 

 they are properly called a gastrovascular system. Indigestible residues 

 are eliminated through the mouth, which thus functions as both mouth 

 and anus. 



62. Gonionemus: Diffusion 



The jelly-like mesoglea is present everywhere between the gastro- 

 dermis and epidermis. In the tentacles it is very thin but in the um- 

 brella it is thick, providing bulk and determining the shape of the 

 relaxed animal. In Gonionemus the mesoglea lacks cells and is nonliving. 

 Since it is about 96 per cent water, dissolved materials diffuse readily in 

 all directions. Diffusion is an adequate mechanism in jellyfish for the 

 distribution of nutrients from gastrodermis to epidermis, and for respira- 

 tion and excretion (cf. Chapter 5). 



