GRAPT0LITE8. 



61 



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

 Sarsia, wliere nerve-fibres extend aroiind the margin and 

 along the radial tubes (Romanes). 



In the groups of CampanularicB, represented by Plumu- 

 laria, Sertularia, Zygodactyla, Dynamena, and Cam2Xtnu- 

 laria, the ectoderm is protected by a horny or chitinous 

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

 tract, when disturbed, into small celh (hydrotheoffi), 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 

 slieaths {gonotheccB) 

 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 ,,. ^„ „ ^ , . ■,. i- «™ 



. FiK. 42.— ScrfiiZaiva ra6tffa>io of Europe, a, natu- 



manv other Hvdroids ralslze; 6, magnifled, showing the hydrarium, with 

 '' • the cells. — From Macallister. 



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. 



Gtraptolites.— More nearly allied perhaps to the Sertularian 

 Hydroids tha.n 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 Oraptolithus Logani the hydroid colony (hy- 

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



