HYDROZOA. 



233 



Th 



Septa but slightly developed. Mzdrepora cervicornis Lam., Dendropliyllia 

 ramea Edw., Mediterranean. Astroides ealycuiaris Pall. 



CLASS II. POLYPOMEDUSJE.* [HYDROZOA.] 



Polyps without cesophageal tube, with simple gastrovascular cavity , 

 The generative elements are developed in medusoid forms which may 

 be either free-swimming, or permanently attached to hydroid 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 Scyphomedusse 

 [Acraspeda], known 

 Scypldstoma, 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- 

 tions of the ectoderm, 

 which as delicate 

 tubes surround the stem and its ramifications, and sometimes form 

 small cup-like structures surrounding the po^yp, and known as 



* Escholtz, " System der Acalephen," Berlin, 1821). Th. Huxley, " Memoir 

 on the Anatomy and Affinities of the Medusae," Phil. Trans., London, IS-l'J. 



FIG. ISO a. Branch of on Obelia-stock (0, ge j atino?a). O, 

 Mouth of a nutritive polyp wi h extended tentacles. M, 

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

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



