OF THE ACALEPHiE OF NORTH AMERICA. 275 



villosities mentioned as characteristic of many Medusae may be owing to a similar 

 epithelium differing in the form of its cells ; but, as I shall have an opportunity of alkid- 

 ing to these cells again when speaking of the Pleurobrachia of the American shores, I will 

 not dwell longer upon this subject now, nor make any allusion to the descriptions given 

 by Ehrenberg of the structure of the tissues of Medusae, as he observed it in the genus 

 Aurelia ; since his statements will better be discussed when describing the species of that 

 family which occur in this vicinity, which will be done in the third part of this 

 paper. The superficial bundles of contractile cells,* which are covered by this epithe- 

 lium, form a remarkable network all over the external surface. They are arranged in 

 two particular directions, the main bundles (Plate III. Fig. 7, S, a, a; Fig. 2, d, d; Fig. 

 1, e,f) being vertical, and alternating with the chymiferous tubes; there being, also, cir- 

 cular bundles (Plate III. Fig. 1, a, a, c, c, h, h ; Fig. 2, a, a) upon the surface, to which the 

 vertical ones are attached, or with which they are at least connected. These transverse 

 or circular fibres form first a circle (Fig. 1 and 2, a, a) above the curves of the chymif- 

 erous tubes, and another (Fig. 1, c, c) somewhat below, though this is less regular than 

 the upper one. The upper crown or circle of muscular fibres appears like a loose net 

 (Fig. 1, b,b) hanging over the gelatinous disk, and its position seems to favor the action 

 of the vertical bundles, which are so connected with the circular fibres as probably to 

 derive support in their action from their simultaneous contraction ; for, from this upper 

 ring there are muscular fibres rising upwards and arched over (6, b) towards the centre : 

 so that, when well preserved, the muscular apparatus of this upper region of the spherical 

 body appears as if covered with arches of muscular bundles converging upward, and resting 

 upon a circular ring below their base. The network of arches upon the most prominent 

 part of the summit of the disk does not exist at all in the specimen of Plate 111. Fig. 2 ; 

 but the circle of fibres, a, a, corresponding to that of Fig. 1, ff, a, is stronger in its devel- 

 opment, and when contracting seems to have been more deeply impressed into the gelati- 

 nous disk, than the more extensive net of muscular fibres of the specimen represented in 

 Fig. 1. Fibres extending downwards {d, d) from this upper crown, in connection with 

 similar transverse fibres below, form the second circle (Fig. 1 , c, c), which, in its com- 

 plete state, constitutes, as it were, a lower series of arches, upon which the upper 

 rest, or to which they are suspended. From this double row of large muscular 

 meshes arise the vertical bundles (Fig. 1, e; Fig. 2, d,d), which extend to the lower 

 margin of the disk, meeting it in the middle (Fig. 1,/), between two sensitive buds. 

 There are, therefore, four of these more prominent external bundles of contractile fibres ; 



* The whole system of contractile cells, in whatever position they occur in the body, is figured upon Plate 

 in., in blue outlines. 



