Chap. III. GENUS BOLINA. 263 



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

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

 the digestive sac itself is so much narrowed immediately above its external opening, 

 that the digestive cavity appears like a loose bag suspended in a mass of trans- 

 parent jelly, widest about half its height, with prominent angles in advance and 

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

 state the coeliac chymiferous tulles have a more curved, and even sinuous course, 

 upon the sides of the digestive cavity, in accordance with the position of the 

 morsels of food within, whilst the upper end of the digestive sac opens freely into 

 the central chymiferous cavity. Along the abactinal end of the digestive sac there 

 are, as in Pleuroln-achia, marked vertical folds, of a l^rownish color, much darker 

 than the transparent walls of the other parts of the sac ; but I have tailed to 

 see distinctly the vil)ratory cilia of its abactinal opening, which play so conspicu- 

 ously about this region in Pleurobi'achia, though there is also in Bolina a constant 

 movement of the minute ^^articles of digested food about the aperture ' leading from 

 the digestive cavity into the chymiferous cavity. 



As mentioned al:)Ove, the central chymiferous cavity and its funnel are not only 

 shorter, but also narrower, in the genus Bolina, than in Pleurolirachia, and the 

 fibrous appearance of the cell walls of that region is very distinct. The actinal 

 part of this cavity has also a somewhat different form from that of Pleurobrachia, 

 though it exhibits the same general disposition, — its sides bulging simply outward, 

 instead of forming two distinct trunks for the branches to the ambulacral tubes, 

 as in Pleurobrachia. The four main Iiranches, from which the eight ambulacral 

 tubes are derived, arise in pairs, almost directly from the main cavity, and, bending 

 slightly sidewa3^s, run almost parallel with one another in opposite directions, that 

 is, forwax'd and backward. The ambulacral tubes themselves present a remarkable 

 arrangement : those of the lateral spheromeres being much further apart from one 

 another than the anterior or the corresponding posterior ones, and diverging side- 

 ways, while those of the anterior and posterior spheromeres follow a more direct 

 course forwai'd and backward, owing, no doubt, to the lateral compression of the 

 body. And from the wide space between the tAVO main branches of one side arise 

 the vertical tubes which descend along the digestive cavity toward the base of 

 the tentacles, as well as the tentacular tubes themselves, the coeliac tubes occu- 

 pying the proximal, and the tentacular tubes the distal side of the lateral inter- 

 ambulacra. 



Again, the four main branches of ambulacral tubes, instead of stretching hori- 

 zontally toward the ambulacra, as in Pleurobrachia, are bent toward the abactinal 

 area, and then divide each into two branches, to provide the eight ambulacra with 

 as many vertical ambiUacral tulDCs. The consequence of this arrangement is, that 



