POLYPI. 



[ 523 ] 



POLYPI. 



processes are found attached to various parts 

 of the polypidoms of some polypes (PL 33. 

 figs. 5 6 * & 26). They consist of a body 

 (fig. 26/), a hinge- or lower-jaw-hke pro- 

 cess (fig. 26 e), and a pedicle or stalk (/). 

 They are attached by the pedicle to the in- 

 terior of a round hollow process, projecting 

 slightly from the sm-face of the polypidom 

 (fig. 26 a). The body is divided by an ob- 

 lique ridge (fig. 26 d) on its inner surface 

 into two chambers. The hinge-process is 

 articulated to the concave surface of the 

 body by a hinge-joint and two articular pro- 

 cesses, and is moved up and down by an 

 elevator and a depressor muscle (fig. 26 c). 

 The body of the processes is hollow, and 

 its concave surface has three apertures. 

 The motion of these processes continues 

 long after the death of the polypes. They 

 appear analogous to the pedicellarise of the 

 Echinodermata. 



Three other structures (PL 33. figs. 5d, 

 5 e, c, a, b) are met with in some polypes, 

 one of which resembles the above in regard 

 to the movement. They exist at the upper 

 and outer angle of the cells, behind the spines 

 which exist there. The uppermost is a 

 hollow process (fig. 5 d, h), with its upper end 

 free and du-ected outwards and forwards, and 

 with a notched aperture at the upper and 

 lower edges, from which a curved filament 

 (fig. 5 d, d) projects. The interior of the 

 process is filled with a contractile substance 

 which moves the filament upwards and down- 

 wards. The second structure consists of a 

 mucronate process, prolonged from the upper 

 and outer angle of the polype-cell (figs. 5 c, 

 h, 5 d, c) ; it is sometimes transformed into a 

 spine. The third consists of a small rounded 

 cavity with a circular aperture (fig. 5 d, a), 

 situated between the bases of the two above- 

 mentioned. 



The structure of the alimentary apparatus 

 varies in the two orders of polypes. In the 

 Anthozoa it consists of a mouth and a simple 

 bhnd gastric sac, the food being admitted 

 and the undigested portion rejected from 

 the single aperture. In the Bryozoa an oral 

 and excretory orifice are present, with an 

 oesophagus, a kind of gizzard, sometimes 

 with horny teeth, a stomach, small and large 

 intestine (PL 33. fig. 18 d, e, f, g). The 

 anal orifice is situated near the mouth, but 

 outside the row of tentacles. 



The oral orifice is usually surrounded by 

 a ring of contractile arms or tentacles, which 

 are hollow internally and communicate with 

 the cavity of the abdomen. They are either 



simple or feathery, and arranged in one or 

 more rows. The food is brought to the 

 mouth either by the tentacles, or by currents 

 induced by the action of their ciha. 



The simple gastric sac of the Anthozoa is 

 usually separated from the cavity of the body; 

 whereby a larger or smaller abdominal 

 cavity is formed, which is almost always 

 prolonged into the hollow arms, and in 

 many polypes hving in colonies passes into 

 the canals traversing the interior of the 

 polypidom, so that the abdominal cavities 

 of the individual polypes are all brought into 

 connexion by these canals. Sometimes lon- 

 gitudinal partitions run hke a mesentery 

 from the outer to the inner surface of the 

 abdominal walls, thus dividing the abdominal 

 cavity into chambers. The bottom of the 

 gastric cavity in some, if not all Anthozoa, is 

 provided with one or more spontaneously 

 closeable openings, by which the gastric 

 cavity communicates with the abdominal 

 cavity. The gastric cavity appears covered 

 with a very delicate cihated epithelium, which 

 is continued through the gastric apertures 

 into the abdominal cavity, and here not only 

 covers the outer surface of the stomach and 

 the septa, but also the inner surface of the 

 abdominal walls, the cavities of the arms, and 

 the canals of the polypidom. 



The walls of the stomach are variously 

 coloured white, yellow or brown, from the 

 presence of aggregations of pigment-cells in 

 the gastric walls (liver-cells), which most 

 probably perform the function of a liver, as 

 there is no glandular appendage correspond- 

 ing to a liver present. 



In the Bryozoa, the inner surface of the 

 digestive canal is covered with ciliated epi- 

 thelium. 



A blood-vessel system has hitherto been 

 found in but few polypes; when present 

 {Alcyonidium, Alcyonium), it consists of lon- 

 gitudinal and circular vessels existing in the 

 abdominal walls, with intermediate cajiillary 

 networks. The vessels have distinct walls. 



A peculiar chculation takes place in almost 

 all polypes, by the to-and-fro motion of a 

 nearly transparent liquid containing minute 

 colourless corpuscles in the abdominal cavity. 

 The liquid ascends thence to the apex of the 

 hollow tentacles, whence it returns to the 

 abdominal cavity. In the colonial polypes 

 this cu'culation continues through the canals 

 which traverse the pohpidoms, from one 

 abdominal cavity to the other. The motion 

 is produced by very delicate ciliated epi- 

 thelium lining the abdominal cavity, the 



