Chap. III. GENUS BOLINA. 255 



materially from the view just descril>ecl, excepting that the mouth is in sight in 

 the centre, extending forward and liackward in the same plane as the circum- 

 scribed area opposite, and the ambulacra appear only indistinctly through the mass. 

 The body is sometimes stretched to so great a degree in the direction of the 

 longitudinal diameter, as to give its outline an irregular, square, oljlong form. How- 

 ever, this attitude is only assumed when the animal swims at the surface of the 

 water, with the mouth turned i;pwai'd. 



Viewed in profile, the Ijody presents also two very distinct aspects : when seen 

 by the broad face or hy the narrow face, or when examined from its anterior or 

 posterior or from its lateral sides. Facing the anterior or posterior end, the sym- 

 metry of the outline {Fiff. 89) arises from the parity and symmetry of the right 

 and left halves of the body, the two sides of the anterior and posterior lobes 

 being perfectly symmetrical. But here again the outlines may differ greatly, in 

 consequence of the expansion or contraction of the lobes, which may hang down 

 and look almost straight with the main mass of the body above, or spread laterally 

 and assume a rounded form, like a broad apron suspended from the chest with 

 projecting auricles or appendages about its point of insertion. In this position the 

 anterior or posterior pairs of ambulacra are seen in their fullest development, 

 extending from the summit along the middle of the lobe to its lower margin, 

 tapering gradually as the lobes grow thinner. Seen from the sides {Fiff. 88), the 

 symmetry of the outline arises from the perfect symmetry and equality between 

 the anterior and the posterior extremity of the body ; but the outlines may vary as 

 the two lobes are pressed nearer together, or stretched apart to a greater or less 

 extent. The modifications in this respect are almost endless, as also are the waj^s 

 in which the margins of the lobes fold over ; for their lower margin may hang 

 loosely down, or it may Ijcnd inward, curving itself in rounded or square outlines, 

 and reaching also over the sides or stretching more flatly. In these various states 

 of dilatation or contraction, the lobes may diverge from each other in all possible 

 degrees : one may even overlap the other alternately, and thus reduce to the utmost 

 the difference Ijetween the longitudinal and the transverse axis. The small lateral 

 lobes, two in number on each side, may, in these various changes of form, assume 

 also the most diversified positions, — at times stretching straight downward, at times 

 arching upward, at times hanging down and converging toward, and even crossing 

 each other ; so that there is no end to the diversity of these aspects. I should 

 say, however, that the motions of these lobes, especially those of the two large an- 

 terior and posterior lobes, are comparatively very slow and graceful ; while those 

 of the small lateral lobes are somewhat more brisk. 



Seen from the sides, the two lateral ambulacra converge from the al)actinal area 

 toward the base of the lateral lobes, and the anterior and 23ostcrior ambulacra of 



