II VDROZOA. 



233 



Th 



Septa but slightly developed. Mudn/ini-i/ cervicornis Lain., I)rnd i-o/ih i/ll.ia 

 ri/ Hint Edw,, Mediterranean, Axt ruiilc-x calycularis Pall. 



CLASS II. POLYPOMEDUS.E.* [HYDEOZOA.] 



Polyps without ajsophayeal tithe, with ximple gastrovascular cavil;/. 

 The generative elements are devduj><'<l. in medusoid forms n-Jtich may 

 be either free-sn'i in in iiiij, <>r jwrnianentltj attached to hi/droid forms. 



This class includes the small polyps and polyp stocks, and the 

 Medusce which form the sexual generation. The Polypomedusce 

 have always a simpler structure than the Anthozoa to which they 

 are also usually infe- 

 rior in size. They 

 lack oesophagus, 

 septa, and gastrovas- 

 cular pouches. Only 

 the polyps of the a- 

 sexual generation of 

 the Scyphomedusa? 

 [Acraspeda], known 

 as Scyphistoma, pos- 

 sess a remnant of 

 the gastric folds as 

 four gastric ridges 

 from which filaments 

 are developed. The 

 polyp stocks develop 

 in rare cases (Mille- 

 poridce) a compact 

 calcareous framework 

 comparable to the 

 polyparium. When 

 skeletal formations 

 are present they con- 

 sist as a rule of more 

 or less horny secre- 



FIG. Iso it. Branch of an Olielia-stnek lO, fieluflnnxa). O, 

 Mouth of a nutritive polyp with extended tentacles. J/, 

 Medusa buds on the body of a proliferous polyp (Vjla.-to- 

 style) ; Th, bell-shaped cup (theca) of a nutritive polyp. 



tions of the ectoderm, 



which as delicate 



tubes surround the stem and its ramifications, and sometimes form 



small cup-like structures surrounding the polyp, and known as 



* Eseholtz, "System cler Acalephen,'' Berlin, 1829. Th. Huxley, "Memoir 

 on the Anatomy and Affinities of the Medusae," Phil. Trans., London, 1849. 



