ZOOPHYTA HYDROIDA. 
83 
the common medullary pulp is retained, (Fig. 9, b.) * Besides 
the cells there are found, at certain seasons, a larger sort of ve- 
sicles, readily distinguished from the others by their size and 
the irregularity of their distribution. — The more robust tribes 
grow erect, and, being flexible and elastic, yield readily to the 
waves and currents ; but some of the very delicate species avoid 
a shock for which they are unequal by creeping along the surface. 
Fig. 9. 
The polypidoms, when dried, are for the most part of a yel- 
lowish or horn colour. 66 When they are immersed in water, 
they recover the same form they appeared in when fresh in the 
sea ; and soon become filled with the liquid. This gives them 
a semitransparent amber colour, and makes them very elastic.”f 
Their material appears to be analogous to horn or condensed 
* Lister, in Phil. Trans. 1834, p. 371. 
f Ellis, English Corallines, p. 3. 
