266 C(ELENTERATA. 



alternation of generations is replaced by polymorphism; it can 

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



16. The scyphostoma and the acraspedote medusa are typical 

 of the SCYPHOZOA. 



17. The scyphostoma differs markedly from the hydroid polyp 

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



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

 edge, gastral tentacles (phacellse), and entodermal gonads. 



19. The medusa arises from the polyp by terminal budding 

 (strobilation). 



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

 suppression of the scyphostoma. 



21. The ANTHOZOA have only one form, the coral polyp; it is 

 distinguished from the hydroid polyp by the ectodermal oesophagus, 

 the radial septa reaching the oesophagus; the well-developed 

 mesoglcea and the gonads which, arising from the entoderm, early 

 migrate into the mesoglcea. 



22 k . Most Anthozoa are colonial and produce a skeleton usually 

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



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

 individual polyps (cortical skeleton). 



24. The living Anthozoa are divided according to the number 

 of septa into Octocoralla and Hexacoralla. To these are added 

 the fossil Tetracoralla. 



25. The Hexacoralla have numerous tubular tentacles and six, 

 or a multiple of six, pairs of septa. 



26. The Octocoralla have eight single septa and eight feathered 

 tentacles. 



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

 large mesoderm with numerous muscle cells. 



28. Nettle cells are absent, and are replaced by adhesive cells. 



29. Most characteristic are the eight meridional rows of 

 ' combs ' whose motions are controlled by a common organ, the 

 sense body, constructed like an otocyst. 



30. The digestive tract consists of an ectodermal oesophagus 

 and a branching system of entodermal vessels. 



