HYDROZOA. 



233 



Septa but slightly develop jd. Midi'/'porn ci-rvicornls Lam., Dcndroplnjllia 

 ramca Edw., Mediterranean. Astroides calycularis Pall. 



CLASS II. POLYPOMEDUSJE.* [HYDEOZOA.] 



Polyps without cesophageal tube, ivith simple gastrovascular cavity. 

 The generative elements are developed in medusoid forms ivhich may 

 be either free-swimming, or permanently attached to hi/droid forms. 



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

 Medusce which form the sexual generation. The Polypomeduscs 

 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 Scyphomedusse 

 [Acraspeda], known 

 as ScypMstoma, pos- 

 sess a remnant of 

 the gastric folds as 

 four gastric riclge.s 

 from which filaments 

 are developed. The 

 polyp stocks develop 

 in rare cases (Mille~ 

 poridce) a compact 

 calcareous framework 

 comparable to the 

 polypaiium. When 

 skeletal formations 

 ai'e present they con- 

 sist as a rule of more 

 or le.-s horny secre- 

 tions of the ectoderm, 

 which as delicate 

 tubes surround the stem and its ramifications, and sometimes form 

 small cup-like structures surrounding the jo'yp, and known as 



FIG. ISO a. Brarch of an Obelia-stock (0, ge'atinofa). O, 

 llnuth of a nutritive polyp wi h extended tentacles. JLT, 

 Medusa buds on the body of a proliferous polyp (blastu- 

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



* Escholtz, "System cler Acnlephen," Berlin, 1S20. Th. Huxley, "Memoir 

 on the Anatomy and Affinities of the Medusas," Phil. Trans., London, 184U. 



