666 SR YOZOA —INFUNDIB ULA TA. 



The circle of tentacles surmounts a conical proboscis, which, together with 

 the tentacles, can be completely withdrawn into a sheath cavity, over which 

 there falls a horny lid. The proboscis is merely a part of the cell wall, which 

 alw^ays remains flexible and capable of being rolled in or out. The horny lid 

 is a specialised fold of the wall, and is characteristic of the order of Bryozoa 

 {Ghilostomata, lip-mouthed) to which Flustra belongs. A glove-finger, with 

 a hole in the top, surrounded by a crown of bristles to represent the mouth 

 and tentacles, will convey an idea of the arrangement of the tentacular sheath ; 

 when the top is rolled in, a fold or lid must be supposed to fall over the cavity 

 thus formed. The tentacles are hollow, and each is provided with a double 

 row of cilia, by whose motion currents are set up, which converge to the 

 mouth. This opening leads into a pharynx and gullet 

 (Fig. 3), and the latter opens into the stomach, which 

 forms the bend of the bent tube; the stomach is continued 

 into the intestine, which opens to the exterior outside the 

 circle of tentacles. The U-shaped intestinal tract is sus- 

 pended in the cavity of the cell, and is separated from the 

 walls by a space filled with fluid and continued up into the 

 interior of the tentacles; the latter organs thus have a 

 respiratory function, since the peri-visceral fluid is brought 

 into close proximity to the sea-water. Further, the ten- 

 tacles are sensory organs capable of perceiving tactile and, 

 „. T_^p perhaps, other sensations. A nerve ganglion is situated 



Pol^jpiile remo™d "■^ 'he upper end of the cell between the pharynx and 

 from cell. intestine. In the walls of each cell are round sieve-like 



B GuHet"^' areas, through which the inner linings of adjacent cells 



c' stomach. become continuous, so that an organic and, perhaps, 



D, Intestines nervous continuity exists between all the cells of a 



E, Vent. 1 ^ 



colony. 



Passing down from the stomach to the base of the cell is a cord, in wLich 

 the eggs and male cells are formed. The fertilised eggs pass up to a hemi- 

 spherical brood-chamber, which forms a hood over the top of some of the 

 cells, and there develop into embryos, which swim away from the parent. 

 The embryos are minute, nearly spherical, bodies, flattened at each pole, and 

 with a broad zone of cilia round the equator. At one pole is the mouth and 

 intestine, and at the other a flattened d.sk, wherewith the embryo, after a 

 short free life, fixes itself, and gives lise to the first poljpide; the latter 

 produces buds which develop, and in their turn produce other buds, with 

 the result that a colony is formed. 



Scattered over the surface of the frond, and interpolated among the ordi- 

 nary cells, are small oval shallow cells -nith large thick lids ; here the polypide 

 has degenerated, and a muscular apparatus for opening and shutting the lid 

 alone remains. The purpose served by these modified cells is the retention 

 of prey, and probably the motion of the lid also serves to frighten away un- 

 desirable intruders. In some species of Bryozoa these metamorphosed cells 

 have an extraordinary resemblance to birds' beaks, and hence were named 

 "avicularia" (amcula, a bird's beak), this name being applied to all such 

 cases of metamorphosis, whatever the shape of the modified cell may be. 



In all Bryozoa we find an approximately U-shaped intestinal tract contained 

 in a sack or box. The mouth is either surrounded by a circular or horse- 

 shoe shaped crown of tentacles, or the area surrounding the mouth is drawn 

 out into processes provided with tentacles, and the vent opening is on about 



