398 The Animal Kingdom 



close relationship of these animals to the coelenterates. In other re- 

 spects, however, they show some advancements : the better organiza- 

 tion of the gastrovascular tract often with an anal pore, the presence 

 of an aboral sensory region, and the mesenchymal muscles. 



The sea walnuts or comb jellies are pelagic forms which are often 

 seen floating at the surface of the ocean. During the day, these animals 

 have an iridescent sheen and at night they are brightly phosphorescent, 

 appearing like glowing orbs on the surface of the subtropical seas. They 

 are characterized by the presence of eight rows of ciliary plates, thus 

 the name sea walnuts. Other forms which live in the deeper parts of 

 the ocean are quite different in appearance. One of these is a creeping 

 form, and is only a few inches in diameter although it is as much as 

 three feet long. This is the Venus's girdle, Cestum. 



Characteristics of the Phylum. — Like the coelenterates, the 

 ctenophores represent a tissue grade of organization. They are some- 

 what more highly specialized than the coelenterates, but no true organs 

 are found. 



The mesoglea is filled with mesenchymal cells derived from the 

 ectoderm and must be considered a true mesoderm. These animals are 

 thus truly triplohlastic. Within the mesoderm are true muscle cells 

 which are derived by transformation from mesenchymal amoebocytes. 



Like the members of the class Anthozoa, the ctenophores are 

 hiradiaUy symmetrical. They lack nematocysts, but characteristic of 

 them are the eight rows of ciliary combs or plates. These originate 

 near the aboral end and terminate just before the oral end. The rows 

 are formed of large numbers of fused cilia which give the appearance 

 of combs. The beat of these combs propels the animal forward. 



Most of the ctenophores have two contractile solid tentacles, which 

 are covered with adhesive cells. These are useful in capturing prey. 



Essentially the nervous system is like that of the coelenterates, 

 inasmuch as it consists of multipolar cells fused into plexuses below 

 the combs. Another plexus forms a ring around the mouth. 



A complex sense organ is present at the aboral end. This is pri- 

 marily a balancing organ and consists of calcareous granules supported 

 on four long, curved, ciliary tufts. There is a transparent covering over 

 the depression in which this is contained. 



Asexual reproduction has never been demonstrated in the members 

 of this phylum. All are hermaphroditic, and many have two periods of 

 sexual activity : one while they are larvae and one when adult. The 



