266 CCELENTEllATA. 



alternation of generations is replaced by polymorphism; it can 

 entirely disappear witli the total loss of either hydroid or medusa. 



16. The scyj)hostoma and the acrasiDedote medusa are typical 

 of the ScYPHOZOA. 



17. The scyphostoma diifers markedly from the hydroid polyp 

 in the presence of four longitudinal gastric folds or septa (tieniolae). 



18. The acraspedote medusa laclvs a velum, has a lobed umbrella 

 edge, gastral tentacles (pliacella?), and entodermal gonads. 



19. The medusa arises from tlie pol5'p by terminal budding 

 (strobilation). 



20. Alternation of generations rarely is lost, and then only by 

 suppression of tlie scyphostoma. 



21. The AxTiiozoA have only one form, the coral jiolyp; it is 

 distinguished from the hydroid polyp by the ectodermal cesophagns, 

 the radial septa reaching the o'sopliagus; the well-developed 

 mesogla?a and the gonads which, arising from the entoderm, early 

 migrate into the mesoglcea. 



22. Most Antliozoa are colonial and produce a skeleton usually 

 of calcic carbonate, but sometimes of ' horny ' substance. 



23. The skeleton may be either axial or it may extend over the 

 individual joolyjis (cortical skeleton). 



24. The living Antliozoa are divided according to the number 

 of septa into Octocoralla and llexacoralla. To these are added 

 the fossil Tetracoralla. 



25. The Ilexacorulla have numerous tulmlar tentacles and six, 

 or a multiple of six, pairs of septa. 



26. Tlie Oclocoralla have eight single septa and eight feathered 

 tentacles. 



27. The Ctenophoka are always free-swimming and have a 

 large mesoderm v.'itli numerous muscle cells. 



28. Xettle cells are absent, a.nd are replaced bv adhesive cells. 



29. Most characteristic are tlie eight meridional rows of 

 ' comljs ' v.liose motions are controlled by a common organ, the 

 sense body, constructed like an otocvst. 



30. The digestive tract consists nf an ectodermal o:sopliagus 

 and a branching system of entodernuil vessels. 



