OF THE ACALEPH^ OF NORTH AMERICA. 



283 



this magnifying power. But from those figures, (and, if the attention is once called to it, 

 Fig. 1 and 4 of Plate I. will show the same,) it may be noticed that there is a partition 

 stretching inwards from the lower margin, terminating with a free edge, through wiiich a 

 broad and free passage is left between the main cavity of the body and the surrounding 

 medium. This partition, which thus almost closes the main digestive cavity and its con- 

 tents from direct communication with the surrounding medium, is highly movable and 

 contractile, and indeed consists chiefly of muscular fibres, covered with a very thin epi- 

 thelium. It is not a mere prolongation of the lower edge of the disk, nor are its contrac- 

 tile fibres a part of the inner circular system of bundles; for it arises from the inner mar- 

 gin of the circular chymiferous tube, and the main nervous cord. It is not gelatinous, as 

 the disk, and though very clear cells surround and protect the chymiferous tubes and the 

 nervous cord of the lower margin, and thicken its walls, nothing of the clear jelly-like 

 substance is seen within the horizontal partition. It is made up, outside, of epithelial 

 cells, and within chiefly of a thick layer of circular muscular fibres, with which alternate 

 some few radiating fibres, which cross the former at right angles, and are more numerous 

 about the eye-specks. In Plate II. Fig. 14, g, g, a portion of this partition near the sen- 

 sitive bundle is represented, with its circular and radiating fibres. In Plate III. Fig. 12, 

 the linear arrangement of the circular fibres is shown, without their natural connection 

 with the radiating fibres. And in Fig. 14 of the same plate, the radiating fibres, d, alone 

 are represented, to show how they converge towards the main bulb. In Fig. 15, again, a 

 portion of the lower margin is figured, to show the connection between the sensitive cord 

 and the circular chymiferous tube, 6, c, the thick wall of the body, d, and the horizontal 

 partition, a. The fragment here represented is seen from outside, laid flat. The natural 

 connection of all these parts may be better ascertained from Plate II. Fig. 17, where rf, d, 

 represent the vertical tubes above the bulbs; a, a, the circular tube; h, b, the nervous 

 cord ; c, c, the horizontal partition ; /, fibres of the main vertical inner bundles ; and e, the 

 outline of the external surface of the wall of the body. The arrangement of the fibres 

 of the partition is very much the same as that of the contractile fibres of the iris ; and 

 its operation also the same, inasmuch as this main orifice is alternately widely opened 



or closely shut. 



These muscular fibres cannot be referred to the same system as that which lines the 

 main cavity, for they are continuous all round the partition, and not attached in any point 

 of their circuit to other organs, but support each other in their connection. The con- 

 traction of these muscular fibres, however, may be combined in such a manner with the 

 contractions of the other muscular bundles, as either to protrude the partition between 

 the sensitive bundles outwards in the shape of a beak, or to retract it into the shape of a 



