OF THK, ACALEPIIvE OF NORTH AMERICA. 285 



edge. The diflTerence between the form of Ilippociene and that of Sarsia will be partic- 

 ularly obvious, when this figure is contrasted with Fig. 4, which represents the disk of 

 Sarsia in a similar section ; where it is seen, that the lower surface of the vault, a, is 

 uniformly arciied, without a central projection ; that the thicker portion of the vault, b, is 

 much thinner than in Hippocrene ; and that the lateral walls, c, are comparatively higher 

 Fig. 2, 3, present two views in vertical section ; in one of which, Fig. 2, the section 

 passes through the main cavity, across its flat walls, in such a manner as to leave two 

 tubes in each half wholly untouched, which thus follow, in their natural position, the inner 

 surface of the main cavity ; and these tubes are represented in the same position in which 

 they also appear in the transverse sections. Fig. 7 and 8. In the other view, as given in 

 Fig. 3, the section passes through the main cavity, in such a manner as to cut through 

 the angles of that cavity, and pass through two opposite tubes, which appear on the sides 

 of the figure. One of the two other tubes, which are placed at right angles with the 

 former, is seen in its natural position, untouched, rising behind the middle of the main 

 digestive cavity, and descending along the wall below the mouth. In both figures, the main 

 digestive cavity is cut into halves ; but in Fig. 2 it appears narrower, as the cut passes 

 through its shorter diameter, without cutting into the projecting angle, and only the open- 

 ings leading to the vertical tubes are seen, as the tubes themselves rest against the walls, 

 which have remained entire ; while in Fig. 3, the section passing through the tubes them- 

 selves, their cavity is laid open in direct continuity with the main digestive sac, the 

 greatest width of which is here laid open. The inner vertical muscular bundles (Fig. 2, a), 

 as well as the outer vertical ones (Fig. 2, 6), and the transverse bundles (Fig. 3, c), as 

 well as the horizontal partition (k, k, k, k), are distinctly shown, the relative size of the 

 fibres being, however, exaggerated, in order to bring more fully into view the different 

 systems. The nervous system is represented by simple dots (Fig. 2) ; d, d, being the 

 transverse section of the main circular cord, and e, e, the transverse section of the upper 

 circular thread. In Fig. 3, however, the vertical nervous threads, f,f, and the plexus, 

 g, g, under the arch of the vertical chymiferous tubes, may be followed along their whole 

 course. The eye-specks, with the tentacles, are also introduced (Fig. 3, h, h) ; but the 

 tentacles are cut short near the base in order not to complicate the figure too much. The 

 circular chymiferous tubes (z, i, i, i) appear in the two figures in a transverse section, 

 being in Fig. 3 somewhat wider at the base of the eye-speck. Fig. 5 and 6 represent 

 precisely the same parts in Sarsia, with corresponding letters, so that a comparison of the 

 figures will best exemplify the difference between the two genera. 



Fig. 7 represents a transverse section just above the main digestive cavity; and Fig. 

 8 represents another transverse section just above the eye-specks, where the transverse 



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