56 SHELL GALLEKY. 
The mouth is situated in the centre of the lophophore, surrounded 
by the circle of tentacles ; and the latter, by the action of their cilia, 
set up currents which convey food to the mouth. 
The mouth leads into a pharynx and gullet, the latter opening 
into a stomach, whence the intestine ascends to terminate in the 
vent opening below and outside the circle of tentacles ; the in- 
testines, in fact, form a U-shaped tube (Figs. 4, 5) suspended in 
the body cavity in the interior of the cell. A cord, the funiculus, 
passes from the stomach to the base of the body-cavity. A small 
nerve ganglion is situated within the upper part of the loop of 
intestine. 
The tentacles, intestines, and other organs constitute the " poly- 
pide," the cell being simply the protective house formed by the 
latter. 
The body-cavity, which contains fluid, is in direct communication 
with the interior of the tentacles, which are hollow, and which act 
as respiratory organs by bringing the fluids of the body-cavity in 
proximity to the water. In Flustra the body-cavities of the cells 
are shut off from each other, but pores and sieves in the partition 
walls allow of the junction of the inner linings of these cavities. 
The male and female reproductive elements are formed in the body- 
cavity. The egg develops in a helmet-shaped brood-pouch, the 
ovicell, situated at the summit of the cell and almost immersed in 
the cell above. The ciliated embryo swims about for a few hours 
and settles down to form the first polypide and cell ; from the 
latter there arise buds which remain attached, and produce other 
buds, till a colony like that of Flustra results. 
Among the ordinary cells are certain smaller cells (Fig. 1, a) 
slightly raised above the general level, different in shape from the 
ordinary kind and with thicker lids. These peculiar cells are termed 
avicularia, and chiefly contain muscles for opening and shutting the 
lid. They arise by modification of the ordinary cells, whereby all 
the organs of the polypide have become atrophied except the muscles. 
The Polyzoa * were so named by Vaughan Thompson, who, in 1820, 
discovered that certain plant-like animals, which had previously been 
classed with the zoophytes, possessed a much higher organisation, 
in that the intestine was separate from the body-cavity and not con- 
tinuous with it as in Sea-Firs, Sea- Anemones, and Corals. In 1834, 
Ehrenberg named the group Bryozoa t or Moss Animals. 
* Polus, many ; zoon, animal. f Bryan, moss. 
