476 EVOLUTION OF ANIMALS Part V 



mesoglea. Their contraction makes the body and tentacles more slender and 

 stretches the comparable processes in the epidermis. Their bases are special- 

 ized for movement and their inner ends contain vacuoles usually filled with 

 particles of food that has been partly digested in the enteron. Glandular cells 

 are abundant about the mouth and in the gastrodermis. In hydra, and more 

 evidently in sea anemones, the cells near the mouth produce mucus. A slippery 

 surface must ease the slide of a struggling water flea into the "stomach" (Fig. 

 23.3). The glandular cells also secrete digestive enzymes. 



Mesoglea. In hydra, mesoglea is noncellular and so thin that in stained sec- 

 tions of the body it appears only as a dark line. This is far from true in jelly- 

 fishes whose bulk and shape are largely due to their mesoglea, but when they 

 are washed up on the beaches and the water evaporates only papery wisps 

 remain. 



Digestion, Respiration, and Excretion. Food is brought to the mouth by the 

 tentacles and drawn into it by contractions of the body. It is partly digested 

 in the enteron by enzymes which reduce it to a semifluid. Any partly digested 

 particles of food which remain are then engulfed by the nutritive muscular 

 cells and digestion is completed within them. Thus hydra employs two methods 

 of digestion, an extracellular one like that of higher animals, and an intra- 

 cellular one like that of an ameba. Finally, the completely digested food is 

 absorbed through the cell membranes and passed on from one cell to another. 

 Indigestible wastes are ejected through the mouth. 



There is no special "breathing mechanism" in hydra. The cells take oxygen 

 from the water or from one another and give off carbon dioxide likewise. 

 There is no transporting fluid such as the blood, and no need of it since the 

 body wall is thin and there is no body cavity to separate the digestive tract 

 from the outer cells. Individual cells eliminate nitrogenous waste but have no 

 contractile vacuoles or other special means of doing so. 



Reproduction. Hydras reproduce asexually by budding or under unusual 

 conditions, by transverse division of the body, and sexually by the fusion of 

 male and female sex cells (Fig. 23.6). 



The buds develop near the junction of the enteron and stalk, when the latter 

 is present. In dioecious (separate sexed) species the individuals produced 

 from buds have the same sex as their parent. In a well-fed hydra, a bud will 

 form and separate from the parent within two or three days. Before it separates 

 there is a free passageway between the enterons of the parent and bud, and 

 food swallowed by the parent may be absorbed by the buds. 



The testes and ovaries develop from formative cells in the ectoderm. During 

 the maturation of the sex cells the number of chromosomes in each one is 

 reduced by half (meiosis) . When the sex cells are brought together the chromo- 

 some number is returned to that of the body cells. 



In the ovary, formative cells are absorbed by the future egg until it becomes 



