54 



POLYPIFERA. 



lypes possessing no distinct muscles to assist 

 in these operations. 



In some species of Bryozoa there are only 

 two sets of opercular muscles, whilst in others 

 one set only is perceptible. 



Alimentary system. In Bowerbankia the 

 whole alimentary apparatus has been minutely 

 described and figured by Dr. Farre.* The ten- 

 tacula are united together at their base to 

 form a circle, in the centre of which is the 

 mouth, and from which descends the oeso- 

 phagus, bulging a little at its commencement, 

 and then contracting and passing down nearly 

 straight to its termination. The parietes of 

 the oesophagus, especially at the upper part, 

 which may be more correctly denominated 

 the pharynx {Jig. 56, a, 1), are thickly 

 studded with minute oval spots, arranged 

 closely in contact with each other. The 

 whole organ appears to be highly irritable, 

 and contracts vigorously when food is intro- 

 duced into it. 



At the termination of the oesophagus is a 

 small distinct cardiac orifice {fig. 56, #, 2) 

 opening into a small globular cavity (3), of 

 singular construction, that appears to perform 

 the office of a gizzard, the parietes of which 

 are thicker than any other part of the ali- 

 mentary canal. This gizzard contains two 

 dark round bodies, placed opposite to each 

 other, from each of which dark lines are seen 

 radiating. In the space between these two 

 dark bodies may be seen a number of squami- 

 form spots, arranged closely in contact, and 

 presenting a beautifully regular tesselated 

 appearance, which, on minute examination, is 

 found to consist of a pavement of gastric 

 teeth. 



The gizzard opens downwards into the 

 true digestive stomach (4), an oblong cavity 

 terminating below in a blunt extremity. The 

 entire walls of the stomach are thickly studded 

 with spots of a rich brown colour. These 

 appear to be hepatic follicles, and to prepare 

 a fluid that tinges the whole organ, as well as 

 its contents, of a rich brown hue. 



From the upper part of the stomach, and 

 by the side of the entrance from the gizzard, 

 arises the intestine (6) by a distinct pyloric 

 orifice (5) that is surrounded by vibrating 

 cilia. The intestine is narrow, and passes up 

 straight by the side of the oesophagus, from 

 which it is entirely separate and free, and 

 terminates by a distinct anal orifice in the 

 delicate parietes of the body, close to the 

 outer side of the tentacular ring. The pari- 

 etesof the intestine are marked with pale spots, 

 and, like those of the whole of the alimentary 

 canal, possess a high retractile power. The 

 animal, when in full vigour, is seen projecting 

 from its cell with the arms extended and the 

 cilia in active operation, the upper part of the 

 body being frequently turned from side to 

 side over the edge of the cell, the extremity 

 of which, from its peculiar flexibility, moves 

 with it. The particles carried to the mouth 

 by the action of the cilia, after remaining a 



* Loc. cit. 



little while in the pharynx, are swallowed by 

 a vigorous contraction of its parietes, and 

 carried rapidly down the oesophagus and 

 through the cardia into the gizzard, which 

 expands to receive them. Here they are 

 submitted to a kind of crushing process, the 

 parietes of the organ contracting firmly upon 

 them, and the two dark bodies being brought 

 into apposition. Their residence, however, 

 in this cavity is only momentary, and they are 

 immediately propelled into the true stomach 

 below, where they become mixed with its 

 contents, which, during digestion, are always 

 of a rich brown colour, being tinged with the 

 secretion of its parietal follicles. 



The food appears to be retained for a con- 

 siderable time in the stomach, and may be 

 seen to be frequently regurgitated into the 

 gizzard, whence, after having been again sub- 

 mitted to its operations, it is returned to the 

 stomach. Here it is rolled about by the con- 

 tractions of its parietes, and at its upper part 

 is frequently submitted to a rotating motion. 

 This rotation of particles is chiefly near the 

 pyloric orifice, and a mass may be often seen 

 projecting through the pylorus into the intes- 

 tine, and rotating rapidly in the direction of 

 the axis of the orifice. This rotation is ef- 

 fected by the action of cilia surrounding the 

 pyloric orifice, which, in very transparent 

 specimens, are distinctly visible with high 

 powers of the microscope. 



The granular matter, after rotating for some 

 time at the pylorus (a provision for prevent- 

 ing its too rapid escape from the stomach), 

 passes into the intestine, where it accumulates 

 in little pellets that distend the parietes of 

 the tube, and it is possible that it may here 

 be still further acted upon by these parietes 

 which have a spotted appearance. 



By the contraction of the intestine the 

 little pellets of excrementitious matter are 

 carried rapidly upwards to the anal orifice, 

 which is seen to open and the little pellet to 

 be tilted over its edge, when it is immediately 

 whirled away from the sight in the current 

 produced by the ciliated tentacles, and the 

 orifice of the tube again contracts. 



The general character of the alimentary 

 canal appears to be similar in all the cilio- 

 brachiate polypes, but in juany genera the 

 gizzard does not exist. 



The anatomy of the animals inhabiting the 

 cells of Flustrts and Escharce differs in some 

 particulars from that of Bowerbankia. In 

 these, the crown of ciliated tentacula is in- 

 serted into the extremity of a kind of pro- 

 boscis, which is itself enclosed in a cylin- 

 drical retractile sheath. From the mar- 

 gin of the opening of the cell arises a 

 membrane equalling in length the contracted 

 tentacles, and serving to enclose them when 

 the animal retires into its abode. The ten- 

 tacula when thus retracted, as was the case 

 in Bowerbankia, are not bent upon them- 

 selves, but are perfectly straight and united 

 into a fasciculus, the length of which, however, 

 is much less than that of the same organs 

 when expanded. 



