OF THE ACALEPHjE OF NORTH AMERICA. 323 



completely shut, it disappears almost entirely, and its position is scarcely marked by 

 any thing more than an indistinct outline, towards which the upper ends of the rows 

 of locomotive fringes converge. When half-way open, or while opening, it assumes 

 an oval form, like a fissure across the body, which becomes gradually more and more 

 elongated, then widens, and finally expands into an ample circular funnel-shaped depres- 

 sion. (Plate II. Fig. 1, 2, 5, 10, and 11 ; Plate III. Fig. 3; Plate IV. Fig. 3 and 5; 

 Plate V. Fig. 3, 4, 5, and 6.) These movements are rather slow, and may be compared 

 to the undulations of a slug or snail adapting its mouth to the form of its food. The 

 changes in Pleurobrachia, however, do not seem to be called forth by the approach of 

 food, but are rather the result of a natural disposition in this animal to be in an atti- 

 tude ready to seize upon its prey. The movements are regulated by powerful bundles 

 of muscular fibres arranged in a very regular manner. At first it would seem as if 

 the whole mass of the body were equally gelatinous and transparent. But upon close 

 examination, and even under a slight magnifying power, the large development of 

 muscular fibres throughout these bodies is readily seen, and explains fully the easy move- 

 ments of these animals, and the readiness with which they change their form. 



The arrangement of these muscular fibres being most easily understood in their 

 connection with the vertical rows of locomotive fringes, and the form and position 

 of the circumscribed area opposite the mouth, I shall begin this illustration by some de- 

 tails upon that apparatus. The vertical rows of locomotive fringes (Plates I. to IV.) are 

 entirely superficial. The fringes themselves (Plate II. Fig. 6) seem to be modified 

 epithelial cells, for whenever, in the progress of decomposition, the epithelium is dissolved 

 in the space between the fringes, it is easy to trace the decomposition of the tissue 

 into this apparatus, even where the muscular fibres immediately below remain unaffected. 

 Each vertical row consists of a great number of isolated, transverse, comb-like fringes, 

 placed one above the other, and movable, either isolately, or in regular succession, or 

 simultaneously. Each comb consists of a large number of thread-like bristles, slightly 

 arched upwards and downwards, of which the middle ones are the longest, tapering 

 gradually sideways, so that the combs are, properly speaking, crescent-shaped, with a 

 straight base, the teeth or fringes of which are movable in quick vibrations, up and 

 down, independently in each comb, and even independently to some degree in each ])or- 

 tion of the same comb, as the middle fringes may be seen to move when the lateral 

 are motionless, and vice versa. But generally all the fringes of one comb act simul- 

 taneously ; but the motion in all the many combs of one row is successive, so that, 

 when the combs are very active, they seem like waves moving up and down in rapid 

 succession along each vertical row, or like the waving spikes in a cornfield agitated by 



