Some of the Lower Metasoans 385 



with one end fastened to and continuous with the capsule wall. The 

 attached end of the tube is covered with a small lid or operculum. From 

 the surface of the nematocyst there is a small protoplasmic projection, 

 the cnidocil, which acts as a trigger for the discharge of the stinging 

 thread. Contact with food or other stimuli will evoke a response on the 

 part of the cnidocil. The actual release of the hollow thread is apparently- 

 due to an increase in pressure within the nematocyst. This results in 

 a sudden eversion of the thread. The hollow threads of one type con- 

 tain a toxic material which is injected into the unfortunate prey. There 

 are many types of nematocysts ; but the main ones are as follows : the 

 adhesive ones or ghdinants which fasten the tentacles to solid objects, 

 the volvent type which wind about the prey, and the stinging type or 

 penetrants which paralyze the prey. The nematocysts are present over 

 most of the body surface, but are concentrated on the tentacles where 

 they may occur in batteries. 



The coelenterates have no specialized cells or structures for respira- 

 tion or excretion. Gaseous exchange simply takes place through the body 

 surface. Digestion is both intra- and extracellular. Within the main 

 portion of the gastrovascular cavity, some gland cells secrete a proteolytic 

 enzyme which breaks up the food. The digestion is then completed in 

 a protozoan fashion within the vacuoles of some cells of the gastrodermis. 



Metagenesis or alternation of generations is another characteristic. 

 An asexual generation alternates with a sexual generation which is 

 morphologically different. The asexual phase, the hydroid form or 

 polyp, is a cylindrical sessile form which may be either solitary or 

 colonial. This hydroid phase or polyp by budding produces the jelly- 

 fish or medusoid stage. This latter is a free swimming bell-shaped 

 animal which produces either eggs or sperm or both. The zygote de- 

 velops into a planida larva which swims about for a while before settling 

 down and metamorphosing into the typical polyp. While this alter- 

 nation of generations is typical, there are many species in which one or 

 the other stage is reduced or suppressed. 



The Classes of Coelenterates. — Three classes are recognized in 

 this phylum. They are differentiated from one another on the basis of 

 the dominant stage in the life cycle and various morphological details. 

 They are as follows : 



Class I. Hydrozoa. In the life cycles of the members of this class there 

 may be polyps, only medusae, or both. The polyp has the gastrovascular cavity 

 undivided by any septa, and the mesoglea is without cells. The medusa has a 



