OF THE ACALEPH.E OF NORTH AMERICA. 279 



visible at all, they are very distinct. Tliis layer lines the whole cavity between the main 

 digestive sac and the lower partition, covering the four radiating chymiferous tubes, and 

 being itself covered by a very thin epithelium, not always easily seen upon every point ; 

 the existence of which, however, is frequently revealed by the polygonal flat cells which 

 are detached from its surface. This muscular layer is not a simple coat lining uniformly 

 the whole cavity of the body. On the contrary, it consists properly of four sets of 

 transverse bundles, extending from one chymiferous tube to another ; so that it may be 

 said that there are four walls of horizontal contractile fibres, beginning above the main 

 digestive cavity, curving over between the arches of the chymiferous tubes, then extend- 

 ing downwards to the horizontal circular tube, and meeting for their whole length upon 

 the inner surface of the vertical tubes. Seen from above, these circular fibres appear 

 like a ring encircling the upper part of the central digestive cavity. This ring has a double 

 outline, as may be seen in Fig. 7, b, c; Fig. 9, 6, c ; and Fig. 4, b,c. Whether this 

 double contour, however, arises from the relation between the circular muscles and the 

 chymiferous tubes, or whether there is a double layer of muscular fibres in the upper 

 part of the disk, or whether the double outline arises from the curve of that part of 

 the inner surface, bringing in each particular position certain fibres more prominently in- 

 to view, I have not been able to ascertain. Though in the main horizontal, or transverse, 

 these fibres are more or less arched upwards and downwards, or outwards. They are 

 arched upwards above between the arches of the chymiferous tubes. They are arched 

 outwards about the middle of the bell-shaped disk, and they are somewhat arched down- 

 wards nearer to the lower margin. The fact of these circular horizontal fibres meeting 

 upon the inner surface of the vertical tubes is important, inasmuch as it is one argument 

 more against the supposition, that the vertical threads which accompany the vertical 

 tubes in their regular course, and which 1 have ascribed to the nervous system, are of 

 a muscular nature. Moreover, it is easy to see that, while the former are either in a 

 contracted or relaxed state, the latter always present the same appearance. 



The natural function of these circular fibres is to reduce the capacity of the central 

 cavity, and hence, when assisting the vertical fibres, to contribute to locomotion. But 

 they may also be subservient to the process of circulation, by the traction they exert 

 upon the radiating tubes. A slight contraction may also contribute to renew the water 

 in contact with that inner surface. The fact, that they line so uniformly the whole 

 central cavity of the body, would lead to such a supposition. Indeed, Medusse whose body 

 is so little active and almost closed as that of Hippocrene need some special means 

 besides the locomotive movements to maintain a regular exchange between the water 

 within and the water without the cavity. I have so often observed the lower of these 



