GRAPTOLITES. 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 Campanularie, 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, when disturbed, into small cells (hydrothee), arranged 
in opposite rows on 
the stalk as in Sertu- avy 
laria (Fig. 42), or 
singly at the ends of 
the stalks, as in Cam- 
panularia, while the 
sheaths (gonothece) 
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 
meduse of these and Fig. 42.—Sertularia abietina of Europe. a, natu- 
many other Hydroids ral size; 6, magnified, showing the hydrarium, with 
the cells.—From Macallister. 
can be collected by a 
towing-net, and emptied into ajar, 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 Graptolithus Logani the hydroid colony (hy- 
drosome) is a long narrow blade, with a row of cells on one 
