OF THE ACALEPHiE OF NORTH AMERICA. 361 



shut, but it forms tlio lower outline of the body when the lobes are entirely open and 

 fully spread. In this attitude the mouth is shut, but the lobes are wide open, to inclose 

 any food that may come within reach ; and whilst dropping fragments of oysters upon 

 them, as they are generally turned mouth upwards, in this extreme state of dilatation, 

 I have sometimes seen the lobes close upon such morsels to secure them, and after- 

 wards the mouth expand and open within to swalloyv the food, the tentacles being 

 alternately drawn out and retracted. 



The visible outline of the digestive cavity changes most remarkably in these various 

 operations. When the mouth is shut, and the digestive cavity is empty, the digestive 

 sac is completely flattened and compressed in the direction of the longer diameter, 

 rising like a tapering funnel towards the central chymiferous cavity above ; that is to 

 say, the fold of the digestive sac which is stretched between the antero-posterior angle 

 of the mouth converges towards the upper extremity of the body, and the flattened walls 

 are pressed upon each other. In this position, the vertical chymiferous tube runs down- 

 wards, and along the middle of the outer surface of the digestive cavity, and reaches, 

 near the lateral margin of the mouth, the sac of the tentacles. But after food has been 

 swallowed, and the mouth is contracted into a more sphincter-like shape, and the digestive 

 cavity so much narrowed above its external fissure, that the cavity of the digestive sac 

 appears like a loose bag widest about half its height, with prominent angles in advance 

 and backwards, swollen also laterally, but tapering above and below. In such a state 

 the vertical chymiferous tubes of the sides have a more curved and even sinuous course, 

 in accordance with the position of the morsels of food within, and the upper end of the 

 digestive sac opens freely into the central chymiferous cavity. 



I have seen distinct indications of fibres in the walls of this sac. There are also 

 marked vertical folds along its upper end, of a brownish color, darker than the transpar- 

 ent walls of the other parts of the sac. But I have failed to see distinctly the vibratory 

 cilia of the upper opening, though there is a constant movement of the minute particles 

 of digested food about the aperture of the digestive cavity leading into the chymiferous 

 cavity. 



As mentioned above, this central chymiferous cavity is not only shorter, but also 

 narrower, in the genus Bolina than in Pleurobrachia, though the fibrous structure of its 

 wall is much more distinct. It has also a somewhat dififerent form, though the same dis- 

 position, its sides bulging simply outwards, instead of forming two distinct trunks for the 

 branches to the ambulacral tubes, as in Pleurobrachia ; so that the four main branches 

 arise in pairs almost directly from the main cavity, the tubes of one side, however, at 

 greater distances from each other than the two corresponding anterior, or the two cor- 



49 



