258 ELEMENTS OF BIOLOGY 



Among the Metazoa it occurs conspicuously in certain Ccelenterata. 

 The essentials, namely the order of sexual and asexual generations 

 in a life cycle is as among plants but the details are quite different. 

 Only one class of the phylum (Hydrozoa) exhibit distinct meta- 

 genesis; because this character in a way marks this class as inter- 

 mediate between it and another class we may review the characters 

 of the Ccelenterata in some detail. 



Ccelenterata in General. Ccelenterata are characterized by 

 the fact that they are diploblastic and by the further fact that all 

 members are equipped with a special sort of defensive mechanism 

 in the ectoderm, consisting of a coiled, spring-like structure with a 

 barbed end, embedded in a cell containing a poisonous substance 

 (p. 100). This structure is called the nematocyst and the cell in 

 w^hich the mechanism is formed is known as a cnidoblast. They 

 are most numerous on the tentacles, which surround the mouth, 

 and not only serve as defensive organs but assist in the capture of 

 smaller animals for food. Cnidoblast cells are very sensitive to 

 contact and rapidly discharge the nematocyst. This impales the prey 

 which is rendered less active by the poison and thus more easily 

 passed into the mouth by the tentacles. 



There are three classes of Ccelenterata; the Hydrozoa, the Scy- 

 phozoa, and the Actinozoa. The most familiar of the Hydrozoa is 

 the fresh water hydra, a simple sac-like organism, consisting of a 

 basal disk, which is usually attached to some object, a two-layered 

 body, containing an endodermal cavity by which the food is di- 

 gested, and an upper end consisting of a series of tentacles sur- 

 rounding a mouth set on an eminence in the centre of the cluster of 

 tentacles. Between the ectoderm and endoderm are some interstitial 

 cells called mesoglea. The animal is usually attached but may be- 

 come detached and carried through the water, or it may turn end 

 over end and thus move slowly from place to place. It has no 

 organs for propelling itself through the water. It reproduces either 

 sexually, the eggs or sperms being formed from mesoglea cells in 



