236 CCELENTERATA 



a supporting layer and a circle of tentacles. In the colonial forms there is 

 usually a cuticular envelope, the perisarc, secreted by the ectoderm. 



13. The craspedote medusa is bell-shaped, with smooth bell margin, its 

 aperture partially closed by a diaphragm-like velum; the gonads are 

 ectodermal. 



14. The medusa 1 arise by lateral budding from the hydroid. 



15. If the medusa remain attached to the parent as a sporosac, alterna- 

 tion 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 (tocnioke). 



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

 gastral tentacles (phacelke), and entodermal gonads. 



19. The medusa arises from the polyp by terminal budding (strobila- 

 tion). 



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

 sion of the scyphostoma. 



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

 guished 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 meso- 

 gloea. 



22. Most Anthozoa are colonial and produce a skeleton (coral) usu- 

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



23. The skeleton may be either axial or it may be outside the indi- 

 vidual polyps (cortical skeleton). 



24. The living Anthozoa are divided according to the number of septa 

 into Octocoralla and Hexacoralla. With the latter the fossil Tetracoralla 

 are allied. 



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 ten- 

 tacles. 



27. The CTENOPHORA 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, a 

 statocyst. 



