OF THE ACALEPH^E OF NORTH AMERICA. 333 



impossible to follow tlic natural movements. Again, unless the parts are placed in such 

 a strictly identical position, those which are in pairs will create confusion, as they may 

 come into various positions presenting apparently a close connection with parts to which 

 they are not all related. Again, the peripheric tubes extending vertically over the 

 whole surface cover so easily the origin of the different trunks arising from the main 

 cavity, that it is indeed very perplexing to trace them all in their true connection. Add 

 to these difficulties the circumstance, that the arrangement of parts, owing to the bilateral 

 symmetry' of the body, appears entirely different when viewed from the side, in profile, 

 and in front, and it will be plain that, unless one keeps in mind two distinct images 

 of the various connections of all these stems and their ramifications, in a front view 

 and in a lateral view, combining them in thought with the rapidity with which such 

 an animal may revolve upon itself, it will be impossible for him to trace for a moment 

 its structure while alive, and he will only have constantly before his eyes the tanta- 

 lizing image of a piece of machinery apparently very complicated, the structure of 

 which he has to decipher while it is moving, but moving almost too fast to allow 

 him to seize the connection of the different parts as they pass along, and which is 

 not onl}^ deranged, but destroyed, the moment it is stopped. It was under such cir- 

 cumstances that I undertook to study the circulation of these animals, and though 

 I succeeded in injecting indigo into their main cavity, and in having it circulate for 

 hours at a time within the body of the same animal before it died, and though I 

 was satisfied that not a particle of the colored liquid had passed into any part of the 

 body into which the liquid before it was colored had not naturally free access, and 

 though it was thus plain to me, that, even after being colored, the circulating fluid 

 continued its normal course, I must say that I never investigated a more difficult subject, 

 never had to devote so much time to the same point, and never taxed my patience to 

 such an extent, as during these investigations. 1 insist upon these details, and state 

 them at full length, because I know that I have now cleared up this subject, and may 

 perhaps induce some other student to go through the long description I am about to 

 give of it, since he can expect to have the matter settled for him. Let us proceed in 

 this description as we should with a minute description of the ramifications of the blood- 

 vessels of some highly organized animal. The difference which exists between the di- 

 gestive cavity and the main cavity of the body will first engage our attention. 



In a front view (Plate III. Fig. 2), when the two tentacles appear right and left, 

 and the plane which passes through the longitudinal fissure of the mouth divides the 

 body into halves, we have before us, on our right, one of those halves of the body, which 

 alternates in its contractions with the other half on the left. It is according to this 



