OF THE ACALEPH^ OF NORTH AMERICA. 367 



EXPLANATION OF THE PLATES. 



The genera Pleurobi-acliia and Buliiia, though apparently closely allied, and dilTcring chiefly in the development of 

 gelatinous lobes in the genus Bolina which do not exist in Pleurobrachia, present peculiarities which require the most 

 careful comparison in order fully to understand their true relations. It has appeared to me, on that account, very de- 

 sirable to mark the different parts in both with corresponding letters, in order to prepare the reader for the closest com- 

 parison, and a true estimation of their different direction in the surrounding medium. The different bearing of the sep- 

 arate rays, the difference in length of their longitudinal and transverse diameters, the inverse position of the mouth in their 

 normal attitude, the apparent transposition of the tentacles, which seem turned towards the circumscribed area in Pleuro- 

 brachia, while in Bolina they accompany the main cavity of the edge of the mouth, are so many features in their structure 

 which cannot be fully appreciated in their respective differences, unless the different species are placed in an identical po- 

 sition when compared. I have, therefore, assigned definite numbers to the different rays or rows of combs and radiating 

 tubes, and marked them with corresponding figures, so as to enable the reader to institute the comparison, cither by 

 inverting Pleurobrachia, mouth downwards, to give it the same position as that which is normal in Bolina, or inverting 

 Bolina, mouth upwards, in order to compare it in the same attitude as that in which we usually observe Pleurobrachia 

 when moving. Bat I may repeat what has already been mentioned above, that occasionally both species may be observed 

 in an attitude the reverse of that which they usually assume. 



Before remarking upon the figures which fill the plates accompanying this paper, it will facilitate such comparisons, and 

 the understanding of the individual figures, if I proceed first with an explanation of the letters with which the different 

 parts are marked, which are precisely the same for all the figures in both genera. 



A, B. Gelatinous lubes encircling the mouth, so large in Bolina as to form the greater bulk of the body. The two 

 lobes, as developed in Bolina, are opposed to each other in the axis of the plane of the circumscribed area and of the 

 month, so that they open and shut against the extremities of the longitudinal fissure of the mouth. 



a. The mouth. It assumes the most diversified outlines when shut or expanded in various ways. 



b. The narrow tube of the anterior part of the alimentary cavity, being a sort of gizzard or special division of the 

 alimentary canal corresponding to the anterior portion of its tract in other animals. 



c. The lower division of the alimentary cavity, performing simultaneously the functions of a stomach and those of a colon ; 

 for upon its walls we observe brown hepatic cells, but at the same time there is at its bottom an opening, through which 

 the substances which have been digested are emptied into the main cavity of the body, d. This whole digestive cavity 

 hangs free in the main cavity, and is truly the analogue of the alimentary sac of Polypi, as observed in Actinia, and may 

 also fairly be compared with the alimentary cavity of Echinoderms, especially with that of star-fishes ; though here the 

 sac may be closed, as it never is in our jelly-fishes, or may empty outside of the animal, while in our Medusae it empties 

 into the main cavity. 



d. The main cavity of the body, in which the products of digestion are circulated, mingled with water. This cavity 

 corresponds truly to the main cavity of Polypi, with this difference, that in Polypi there are only partitions dividing it off 

 around the periphery, while in Medusa; the gelatinous mass of which the body consists fills, to a great extent, the 

 inner space of the animal, and leaves only tubes for the peripheric circulation of the fluid contained in it. There is one 

 vertical prolongation,/, of this main cavity, extending in the direction of the circumscribed area, and which branches into 

 two forks,/',/", at its termination. The other tubes arising from it are the two main chymiferous horizontal tubes, e, e, 

 with their branches, q, q, and their eight terminal forks, i, i, which open into the vertical tubes, v, v. The tubes r, r, 

 which follow the walls of the digestive cavity, arise also from it near the main horizontal trunks, and from these latter 

 arise the tubes of the tentacular apparatus, a, a. 



e. e. The main horizontal trunks of the chymiferous tube, from which arise the eight radiating branches. 



