OF THE ACALKIMI.E OF NORTH AMERICA. 351 



that tlie antero-postoiior diameter is tlio longer, while it is the shorter in Pleurohraehia, 

 and vice versa, that the transverse diameter is the shorter, while it is the greater in Pleu- 

 rohraehia. This inverse agreement hetween the natural relations of organs and external 

 form is most satisfactorily ascertained, upon comparing the position and direction of the 

 circumscrihed area and of the tentacles, and we shall see hereafter that in every respect 

 the proportions of the hody with reference to their longitudinal and transverse develop- 

 ment are reversed in the two genera. Before this contrast had heen estahlished, I was 

 unahle to trace the homology of parts between the two genera. Indeed, taking the 

 general form as a guide, I began with comparing the two animals in a position in which 

 I undertook, to place their prominent diameters in the same relation, and thus arrived 

 at the conclusion, that the tentacles, which are far less developed in Bolina, and issue 

 from the margin of the mouth itself, were organs differing from the tentacles of Pleuro- 

 hraehia, which I considered as a system entirely peculiar to that type of Beroe, while 

 the tentacles of the type of Bolina a[)peared to me as a sort of fringes of the mouth. 

 But the moment I placed the diameters of tiie two bodies in a ])ositi()n inverse to their 

 length, all parts being placed in the same natural relation as far as they correspond by 

 structure, their perfect homology throughout the system was at once established. And 

 not only the correspondence and antagonism between the anal area and the tentacles, 

 but also the minor details in the ramifications of the chymiferous system, agreed in every 

 respect. The difficulty under which I had labored was precisely that of an artist at- 

 tempting in a family picture to bring out the resemblance between two kindred faces, 

 while contemplating one individual in profile, and the other in a front view, but believing 

 their position to be the same. With tiiis inverse relation between the homologous parts 

 considered in their reference to form in the two genera Bolina and Pleurohraehia, there 

 is a corresponding opposition between the natural positions of the two animals in the 

 surrounding media. Pleurohraehia, as I have stated, swims naturally with the mouth 

 upwards or forwards, and the anal area downwards or backwards ; in Bolina the an- 

 imal moves with the mouth downwards and the anal area upwards. 



The position of the tentacles, their natural relations to the body when in motion, and 

 the direction of the aperture through which they issue, were the chief sources of error 

 which led me first to consider them as different apparatus ; for in Pleurohraehia (Plate I.) 

 they arc turned downwards towards the anal apertures, while in Bolina (Plate VI.) they 

 are turned downwards towards the oral aperture. But now we may ascertain the homo- 

 logical identity of these appendages, by placing these two animals in the same structural 

 position. It will be easy to understand how, in accordance with the form and movements 

 of the various members of the whole family, the tentacles may issue from different heights 



