LOWER ANIMALS 



199 



Coelenterates have two tissue layers, an outer and 

 an inner. Between the two is material which is mostly 

 jelly and in some forms approaches the status of a 

 third tissue layer. Among these layers are several 

 specialized types of cells. There are special outer 

 layer cells which form the thread cells {nemalocysts) 

 which by stinging, tangling, or other means are used 

 in food capture and defense. Certain T-shaped cells 

 located in the outer and inner layers provide pro- 

 tective and muscle-like tissues — contractile fibers 

 within the cross arm of the T— are responsible for 

 the movements of the body. Of the other cell types 

 found, perhaps the nerve cells are most interesting. 

 These nerve cells are unique; many of them connect 

 into the so-called "nerve net" in which nerve im- 

 pulses pass in both directions over any individual 

 nerve fiber, a phenomenon unknown elsewhere in the 

 animal kingdom. 



Coelenterate reproduction may be asexual by 

 budding, or sexual. Very often the life history is a 

 regular succession of sexual and asexual reproduc- 

 tion, with the sexual generation represented by the 

 medusoid form and the asexual by the polyp form. 

 The hydrozoan Obelia is an example of a species dis- 

 playing this alternation of generations. The fertilized 

 egg develops into a ciliated larva. Settling to the 

 bottom, the larva differentiates and grows to a polyp 

 form from which a whole colony arises by budding. 

 Some of the specialized individuals in the polyp 

 colony give rise to medusae by budding. These 

 medusae are male or female and generally all the 

 medusae produced by one colony are of one sex. 

 The medusae float free in the water and enter the 

 sexual stage by producing eggs or sperm. Slightly 

 different life cycles occur in other coelenterates 

 and represent condensations through loss of some of 

 the stages or dominance of others. 



CLASS HYDROZOA 



Diagnosis: solitary or colonial; both asexual 

 polyps and sexual medusae present, but one form 

 often is suppressed; polyps without vertical mem- 

 branes partitioning the gastrovascular cavity; me- 

 dusae with a velum (a shelf-like membranous ring 

 attached to the inner margin of the "umbrella"); 

 almost entirely marine, but there are a few fresh- 

 water species (the only nonmarine coelenterates); al- 

 though not rare, the fresh-water species tend to be 

 overlooked (Figure 12.6). 



ORDER HVDROIDEA (Hydroids) 



Diagnosis: solitary or colonial plant-like animals; 

 polyp generation well developed, medusae if present 

 are small; marine and fresh water. 



These animals give the appearance of sedate, plant- 

 like creatures. Although the purely reproductive in- 

 dividuals of any colony appear harmless, other indi- 

 viduals of colonies and the solitary species are pred- 

 ators of the microscopic world. These feeding in- 

 dividuals use their thread cells to spear or "rope" 

 their prey and then, using their tentacles, collect 

 their potential food and carry it to be devoured by 

 the mouth. 



Most hydroids are found growing on or among 

 seaweeds. Many kinds can be encountered in inter- 

 tidal areas, perhaps the most conspicuous being the 

 feather-like colonial species called sea plumes. 



ORDER HYDROCORALINA (Hydrocorals) 



Diagnosis: colonial, with the polyp form dominant 

 but also with a massive, calcareous skeleton of coral 

 through which two kinds of polyps protrude; marine. 



Although their behavior resembles that of the hy- 

 droids, hydrocorals are not as likely to be en- 

 countered, because most are deep-water forms. How- 

 ever, they are fairly common along the Gulf of 

 Mexico. In Florida, the stinging coral (Millepora) 

 is attached to reefs and is of concern to swimmers; 

 this hydrocoral possesses powerful stinging cells. 

 Along the coast of California, there is a purple 

 hydrocoral (Stylantheca) that sometimes can be found 

 encrusting rocky ledges at very low tide levels. 



None of these animals are true corals. True corals 

 are members of the Class Anthozoa. 



ORDER TRACHYLINA (Hydromedusae) 



Diagnosis: polyp generation reduced or absent, 

 the medusa being the dominant form; many do not 

 possess any polyp stage, the stage prior to the medusa 

 being a parasite on the parent or another species of 

 hydromedusan; marine and fresh water. 



These predators of microscopic animals and pro- 

 tistans are seldom found by amateur collectors. 

 However, in quiet waters on overcast days one might 



