ORAPTOLITES. 



61 



there is no trace of a nervous system such as exists in 

 Sarsia, where nerve-fibres extend around the margin and 

 along the radial tubes (Romanes). 



In the groups of Campamclarim, represented by Plumu- 

 laria, Sertularia, Zygodactyla, Dynamena, and Campanu- 

 laria, the ectoderm is protected by a horny or chitinous 

 sheath (perisarc) enveloping the zooids. The Hydroids re- 

 tract,-vvhen disturbed, into small cells (hydrothecce), arranged 

 in opposite rows on 

 the stalk as in Sertu- 

 laria (Fig. 42), or 

 singly at the ends of 

 the stalks, as in Cam- 

 panularia, while the 

 sheaths (gonothecm) 

 protecting the medu- 

 sa-buds are distin- 

 guished by their 

 much larger size and 

 cup-shaped form. 



The Sertularians 

 abound on sea- weeds, 

 and may be recogniz- 

 ed from their resem- 

 blance to mosses. 

 They are among the 

 most common objects 

 of the seaside. The 

 medusse of these and 

 many other Hydroids 

 can be collected by a 

 towing-net, and emptied into a jar, where they can be de- 

 tected by the naked eye after a little practice. 



Graptolites.— More nearly allied perhaps to the Sertularian 

 Hydroids than any other known animals are the Graptolites 

 (Fig. 43), which were most abundant in the Lower Silurian 

 period, and lingered as late as the Clinton epoch of the Upper 

 Silurian. In OraptolUhus Logani the hydroid colony (hy- 

 drosome) is a long narrow blade, with a row of cells on one 



Sertularia abletina of Europe. A, natural 

 6ize; B. magnified, showing the hydrarium, with tlie 

 cells. — From Macalliwter. 



Fig. 42.- 



