30 MARINE ANIMALS OF MASSACHUSETTS BAY. 



before its ramifications reach the surface, thus making in all 



eight radiating tubes. So far, these 

 eight tubes are horizontal, all diverging 

 on the same level ; but as they reach 

 the periphery each one gives rise to 

 a vertical tube, running along the sur- 

 face of the body from pole to pole, just 

 within the rows of locomotive fringes 

 on the outer surface, and immediately 

 connected with them (Figs. 27, 28). As 

 in all the Ctenophorae, these fringes keep 



up a constant play of color by their rapid vibrations. In Pleuro- 

 brachia the prevailing tint is a yellowish pink, though it varies to 

 green, red, and purple, with the changing motions of the animal. 

 We have seen that the vertical tubes between which the digestive 

 cavity is enclosed, start like the cavity itself from that pole of the 

 body where the mouth is placed, and that, as they approach the 

 opposite pole, at a distance from the mouth of about two thirds 

 the whole length of the body, they unite in the canal, which then 

 extends to the other pole where the eye-speck is placed. As it is 

 just at this point of juncture between the tubes and the canal 

 that the two main horizontal tubes arise from which all the 

 others branch on the same plane (Figs. 27, 28), it follows 

 that they reach the periphery, not on a level with the pole op- 

 posite the mouth, but removed from it by about one third the 

 height of the body. In consequence of this the eight vertical 

 tubes arising from the horizontal ones, in order to run the entire 

 length of the body from pole to pole, extend in opposite direc- 

 tions, sending a branch to each pole, though the branch running 

 toward the mouth is of course the longer of the two. The tenta- 

 cles have their roots in two sacs within the body, placed at right 

 angles with the split of the mouth. (Figs. 27, 30.) They open 

 at the surface on the opposite side from the mouth, though not 

 immediately within the area at which the eye-speck is placed, 

 but somewhat above it, and at a little distance on either side of it. 

 The tentacles may be drawn completely within these sacs, or be 

 extended outside, as we have seen, to a greater or less degree, and 

 in every variety of curve or spiral. 



Fig. 30. Pleurobrachia seen from the extremity opposite the mouth. 



