470 POLYZOA. 



(1220.) The anatomy of these Polyzoa differs slightly from that of 

 Bowerbankia. The crown of ciliated tentacula is inserted into the ex- 

 tremity of a kind of proboscis, which is itself enclosed in a cylindrical 

 retractile sheath. From the margin 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. These appendages, 

 thus retracted, are not bent upon themselves, but perfectly straight and 

 united into a fasciculus, the length of which is nevertheless much 

 shorter than that of the same organs when expanded. 



(1221.) By the opposite extremity to that fixed to the margin of the 

 opening of the cell, the tentacular sheath unites with a tolerably capa- 

 cious tube, the walls of which are exceedingly soft and delicate ; and 

 near the point of their union we may perceive a fasciculus of fibres 

 running downwards to be inserted upon the lateral walls of the cell : 

 these fibres appear to be striated transversely, and are evidently mus- 

 cular ; their use cannot be doubted. When the animal wishes to expand 

 itself, the membranous sheath above alluded to becomes rolled out- 

 wards, everting itself like the finger of a glove as the tentacles advance. 

 The muscular fasciculi are thus placed between the everted sheath and 

 the alimentary canal, and by their contraction they must necessarily 

 retract the whole within the cell. 



(1222.) The first portion of the alimentary tube is inflated, and much 

 wider than the rest ; it forms a kind of chamber, in which the water set 

 in motion by the vibration of the cilia upon the tentacles appears to 

 circulate freely. The walls of this chamber are extremely delicate : the 

 soft membrane forming them is puckered, and appears traversed by 

 many longitudinal canals united by minute transverse vessels ; this 

 appearance, however, may be deceptive. 



(1223.) Beneath the first enlargement, the digestive apparatus be- 

 comes narrower, but immediately expands again, and offers at this point 

 a certain number of filiform appendages, which appear to be free and 

 floating in the interior of the cell. To the second cavity succeeds a 

 narrow canal, opening into a third dilatation, generally of a spherical 

 form. From the last-named viscus issues a kind of intestine, which 

 soon bends upon itself and becomes attached to an organ of a soft and 

 membranous texture, having the appearance of a caBcum, and which 

 seems to be continuous superiorly with the digestive tube ; the latter 

 continues its progress towards the upper part of the cell, and ultimately 

 terminates by a distinct anal aperture upon the upper aspect of the ten- 

 tacular sheath. 



(1224.) The operculum which closes the cell in Flustrce and Escharce 

 is moved by two muscular fasciculi inserted into the internal face of this 

 valve by the intermedium of two filaments analogous to tendons ; by 

 their inferior extremity these muscles are attached to the walls of the 

 cell ; and when, by its own elasticity, the operculum is turned back, 



