TUBULARIANS. 67 



Sertularians (^BypJiasia rosacea Ag.), abundant on all rocks in 

 sheltered places immediately below low-water-mark, is repre- 

 sented in Pig. 86. In both these Sertularians the Medusae wither 

 on the stock, never becoming free. The free Medusae of the 



Fig- 86. Jig. 87. 



Sertularians are only known in their adult condition in a single 

 genus, which is closely allied to Mehcertum, and which is pro- 

 duced from a Hydroid genus called Lafoea. Fig. 87 repre- 

 sents one of these young Sertularian Medusae (Lafo&a comuta 

 Lamz.). 



In the Sertularian and Campanularian Hydroids we have found 

 that the communities consist generally of a large number of small 

 individuals, so small, indeed, that it is hardly possible at first 

 glance to distinguish the separate members of these miniature 

 societies. Among the Tubularians, on the contrary, the commu- 

 nities are usually composed of a small number of comparatively 

 large individuals ; and indeed these Hydroids may even grow 

 singly, as in the case of the Hybocodon (Fig. 104), which attains 

 several inches in height. There is also another general feature 

 in which the Tubularians differ from both the other groups of 

 Hydroids. In the latter, the horny sheath which encloses the 

 stem extends to form a protecting calycle around the Hydra 



Fig. 86. Dyphasia rosacea, nataral sise. 

 Fig. 87. Medusa of Lafoea. 



