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 belhshapcd, with smooth bell margin, its 

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

 ectodermal. 



14. The medusa? arise by lateral Ijudding from the hydroid. 



I V If the medusa remain attached to the parent as a sporosac, alterna- 

 tion of generations is replaced Ijy polymorphism; it can entirely disappear 

 with the total loss of either hydroid or medusa. 



16. The scyphostoma and the acraspedote medusa arc typical of the 



SCYPHOZOA. 



17. The scyphostoma differs markedly from the hydroid polyp in the 

 presence of four longitudinal gastric folds or septa (taniiokv). 



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

 gastral tentacles (phacelkv), and entodermal gonads. 



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

 tion). 



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

 sion of the scyphostoma. 



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

 guished from the hydroid polyp by the ectodermal cesophagus, the radial 

 septa reaching the esophagus; the well-developed mesoglcca and the 

 gonads which, arising from the entoderm, early migrate into the meso- 

 gk-ea. 



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

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



23. The skeleton may ]>e either a.xial or it may be outside tlie indi- 

 vidual polyps (cortical skeleton). 



24. The li\"ing Anthozoa are divided according to the number of sepia 

 into Octocoralla and Ile.xacoralla. With the latter the fossil Tetracoralla 

 are allied. 



25. The Hcxacoralla have numerous tubular tentacles and six, or a 

 multiple of six, pairs of septa. 



26. The Ocloavalla haw- eight single septa anil eight feathered ten- 

 tacles. 



27. The Cti:nopiior.\ are always free-swimming and have a large 

 mesoderm with nunierous muscle cells. 



2S. Nettle cells are absent, and are replaced by adhesixe cells. 



2q. Most characteristic are the eight meridional rows of 'combs' 

 whose motions are controlled by a common organ, the sense body, a 

 statocyst. 



