103 
are placed alternately, in two rows, their cha- RQQM XI 
racters consisting in the different forms of the 
ceils. In Gemicellaria the cells are equally dis¬ 
posed in two rows; they are opposite, and 
together form an ovate joint. In the genus 
Tricellaria the cells are disposed in three rows, 
whilst in Eucratea , Catenicella , and Menipea the 
cells are placed one above the other, in a single 
unilateral series; these genera differing in the 
position of the mouth of the cells. 
The Polypiaria, which are characterized by 
the horny nature of their coral, are also pecu¬ 
liar for the cells which the animals inhabit 
being united together by a common tube, and 
by their being also formed with external ve¬ 
sicles, whose structure is not distinctly known, 
but which are usually considered as ovaria. 
According to the observations of Cavolini, 
Thompson, and others, the structure of the 
animals of these corals is far more complicated 
than their minute size would lead one to sup¬ 
pose. 
The Tubularice are known immediately 
amongst these corals by the long, tubular, cy¬ 
lindrical shape of their cells. They are to be 
distinguished from the eggs of the marine 
Mollusca, with which they have been con¬ 
founded by Esper, who has even figured the* 
Bysus of the common Muscle as a species of* 
this 
