DIVISION OF LABOR IN THE COELENTERATES 



183 



and the foot. This network of cells lies in the ectodermal layer of 

 the animal, and receives impulses from sensory cells as well as trans- 

 mitting them to the muscle fibrils. The sensory cells of the ectoderm 

 .vary in their location ; one type occurs on the tentacles, one on the 

 hypostome, and a third on the foot 

 (base). Neuro-sensory cells which 

 are located in the mid-body area 

 .also resemble nerve cells, except that 

 they send processes to muscle fibrils 

 and so become intermediate between 

 those receiving stimulation and those 

 making the response. Some nerve 

 cells appear in the endodermal layer 

 but are not, so far as can be deter- 

 mined, connected with the ecto- 

 dermal nerve net. 



Reproduction 



Probably the most important 

 function of the interstitial cells is 

 their growth into sex cells. Most 

 Hydras are hermaphroditic, that is, 

 have both kinds of sex cells present 

 in the same individual, but since the 

 sperm cells and ova ripen at different 

 times, fertilization is accomplished 

 by sex cells from different indi- 

 viduals. Sperm cells are produced 

 by the mitotic division of interstitial 

 cells, each of which first produces a 

 number of parent male cells, contain- 

 ing the somatic number of chromosomes. 



The nerve net in a young hydra 

 as seen with an intravitani methylen- 

 blue stain. Note the ringlike ar- 

 rangement in hypostome and foot. 

 What effect might such an arrange- 

 ment have on movement? (After 

 J. Ilodzi.) 



These cells divide four 

 times and in the process a reduction division takes place, leaving the 

 sperm cells with just half as many chromosomes as the body cells. 

 A somewhat similar process takes place in the formation of the ova. 

 One interstitial cell becomes larger than the others, rounds into a 

 sphere, and is surrounded by other interstitial cells, which serve as an 

 ovary for the growing egg. The latter continues to grow in size, form- 

 ing yolk from the surrounding cells. Just before the egg becomes 

 mature, the process of maturation takes place (see page 429), dur- 

 H. w. H. — 13 



