OF THE ACALEPHjE OF NORTH AMERICA. 331 



wards one or the other extremity, and assuming, therefore, alternately a clavate, or 

 fusiform, or filiform appearance, very much elongated, but nevertheless sufficiently 

 characteristic to be compared to cells, their nucleus being often still distinguishable, 

 but presenting no appearance whatever of striae across the fibre. From tlieir appear- 

 ance, and from the change of their form during contraction, I can hardly doubt that they 

 are hollow, and contain fluid. The chief difference in the arrangement of these fibres 

 in Beroid and Discoid Medusa? consists in the circumstance, that in the former the mus- 

 cular fibres seem to pervade the whole substance of the gelatinous matter which con- 

 stitutes the main mass of the body ; and, indeed, it seems to me that this gelatinous mass 

 is, in its elementary structure, identical with the contractile tissue which pervades it, with 

 only^his dilTerence, that the cells which contain the jelly are less contractile, and their 

 liquid contents more consistent. But that even these parts are not altogether deprived 

 of the power of contraction and dilatation would seem to be conclusively shown by 

 the circumstance of the whole sphere in the same individual appearing at times larger 

 than at others. And if this is so, we have here a body made up, to a very great extent, 

 of the same elements as the foot of a mollusk, for instance, but differing in so far as that 

 there the cells are metamorphosed into more perfect muscular fibres, while here the 

 elements of the cells preserve more of the primitive character of such structures, though 

 they undergo also a peculiar modification, inasmuch as we have here an elastic jelly 

 interspread with contractile fibres. 



I have been unable to distinguish in Pleurobrachia a special superficial system of 

 vertical fibres distinct from those within ; nor are the circular fibres so exclusively in- 

 ternal as in Discoid Medusae, but interwoven with vertical fibres throughout the thickness 

 of the walls of the body, though the vertical fibres are generally more numerous in the 

 outer part of the main mass, and the circular fibres towards the inner part ; as may 

 be seen when examining the mouth in its contracted condition, when the radiating fibres 

 alone are visible externally, as in Fig. 5 and 11, while the circular fibres are chiefly 

 shown when the mouth is fully extended, as in Fig. 10. Though the contractile fibres 

 maintain throughout the body chiefly these two directions, it were an exaggeration to 

 imagine that the fibres all run strictly in either one or the other of these directions. 

 On the contrary, in each bundle or row of either vertical or circular fibres, we find 

 that their course diverges more or less from the prevailing direction, and that, for in- 

 stance, towards the upper and lower summit of the body, about the height at which 

 the vertical rows of locomotive fringes terminate on the two sides, the chief vertical 

 bundles of muscular fibres diverge to form somewhat pennate bundles of fibres, as in 

 Fig. 4 and 5, and that along the sides of the vertical rows of locomotive fringes the 



