POLYZOA ; BOWERBANKIA. 



437 



they are almost always parasitic upon other marine formations, 

 such as corallines, sea-weeds, shells, or even different species of 

 their own tribe. 



1071. The Bowerbankia, a form in which the isolation of the 

 animals and their cells is most complete, will perhaps, from this 

 circumstance and the comparatively large size attained by the 

 species, afford the best means of understanding the structure of 

 the Polyzoa ; and it will therefore be convenient to enter first 

 upon its consideration. The animals of the Bowerbankia densa 

 are, when fully expanded, about half of an inch in length ; when 

 retracted, they are completely inclosed in delicate horny cells, so 

 transparent as to admit of the whole structure 

 being seen through their walls.- These cells, 

 which, when the edges are turned in by the 

 retraction of the animal, are not above half 

 the length of the expanded body, arise separ- 

 ately from creeping stems, which attach them- 

 selves to Flustrce in aggregated masses, of 

 from half an inch to one inch in diameter. 

 The animal possesses ten tentacula, arranged 

 round the mouth ; and each of these is thickly 

 set with cilia on both sides. The mouth does 

 not lead at once, as in the true Polypes, into the 

 stomach ; but it forms the entrance to a wide 

 funnel-shaped tube, which may be termed the 

 pharynx. This soon contracts into a narrower 

 canal, the oesophagus, which terminates at its 

 lower end in the digestive cavities. The first 

 of these is an organ which seems closely to 

 resemble a gizzard. It is of a globular form, 

 And has two dark spots upon its sides, from 

 which radiating lines are seen. These are probably composed 

 of muscular fibres, the office of which is to effect the trituration 

 of the food, by means of the teeth that project from the inner 

 wall of the cavity. The gizzard opens at its lower end into a 

 larger bag, which seems to be the true digestive stomach. Its 

 walls are thickly studded with spots of a rich brown colour; 



FIG. 704. BO-WTER- 

 BLAXKIA. a, oesophagus ; 

 6, gizzard ; c, stomach ; 

 d, orifice of intestine. 



