56 SHELL GALLERY. 



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 Bryon, moss. 



