PHYLUM COELENTERATA (CNmARIA). SIMPLE TISSUE ANIMALS 



117 



upon it. If a hydra remains turned inside 

 out, the cells of the epidermis and gastro- 

 dermis migrate past each other through the 

 mesoglea until they regain their original 

 positions. 



Regeneration may be defined as replace- 

 ment of lost parts. It takes place not only 

 in the hydra, but in many other coelen- 

 terates, and in some of the representatives 

 of almost every phylum of the animal king- 

 dom. The hydra, however, is one of the 

 types that have been most widely used for 

 experimentation. Pieces of the hydra that 

 measure only six thousandths of an inch in 

 diameter are capable of becoming entire 

 animals. The tissues in some cases restore 

 the lost parts by multiplication of their cells; 

 in other cases, they are worked over directly 

 into a new but smaller individual. 



Grafting 



Parts of one hydra may easily be grafted 

 upon another; in this way many bizarre ef- 

 fects have been produced. Parts of two 

 hydras of different species have also been 

 united successfully. 



Space v/ill not permit a detailed account 

 of the many interesting questions involved 

 in the phenomena of regeneration and graft- 

 ing, but enough has been given to indicate 

 the nature of the process. The ability to 

 regenerate lost parts is obviously of benefit 

 to the animal. Such an animal, in many 

 cases, will succeed in the struggle for exist- 

 ence under adverse conditions. Regeneration 

 takes place continually in all animals; for ex- 

 ample, new cells are produced in the epi- 

 dermis of man to take the place of those 

 that are no longer able to perform their 

 proper functions. Both internal and external 

 factors have an influence upon the rate of 

 regeneration and upon the character of the 

 new part. Temperature, food, light, gravity, 

 and contact are some of the external fac- 

 tors. In man, various tissues are capable of 

 regeneration; for example, the skin, muscles, 

 nerves, blood vessels, and bones. Lost parts. 



however, are not restored in man because 

 the growing tissues do not coordinate prop- 

 erly. A decrease in regenerative power seems 

 to be correlated with the increase in com- 

 plexity of animal types. The inability of the 

 more complex forms to replace lost parts 

 appears to be the price of specialization. 



OTHER COELENTERATA 



As in the case of the hydra, coelenterates 

 in general are diploblastic and possess nema- 

 tocysts. Contractile fibers are present in a 

 more or less concentrated condition. Nerve 

 cell processes ( fibers ) and sensory cells are 

 characteristic structures; they may be few 

 in number and scattered, as in the hydra, 

 or numerous and concentrated. Tlie two 

 principal types of coelenterates are the polyp 

 and the jellyfish or medusa. These are 

 fundamentally similar in structure, but 

 are variously modified. Both polyps and 

 medusae are radially symmetrical. Although 

 the medusae may, upon superficial examina- 

 tion, appear to be very different from the 

 polyps, they are constructed on the same 

 general plan. Both have similar parts, the 

 most noticeable difference being the enor 

 mous quantity of mesoglea present in the 

 medusa. The water content of a medusa is 

 very high; that of Aurellia is about 96 per 

 cent. 



Digestion in coelenterates is both extra- 

 and intracellular; enzymes are discharged 

 into the gastrovascular cavity for maceration 

 of food organisms. Tlie particles are trans- 

 ported to various parts of the body by cur- 

 rents in the gastrovascular cavity and are 

 then taken up by the gastrodermal cells and 

 passed over to the epidermal cells. Both 

 respiration and excretion are performed by 

 the general surface of the epidermis and 

 gastrodermis. There is no endoskeleton, but 

 the stony masses built up by the coral polyps 

 support the soft tissues to a certain extent. 

 The nervous tissue and sensory organs pro- 

 vide for perception of various kinds of 



