228 CTENOPHORiE. Part II. 



of the system of tubes which arise from the main cavity and branch into the 

 right and left halves of the body alternate constantly in their contractions, — so 

 much so that the one may be in the state of fullest expansion when the other 

 is in the most complete state of contraction ; and after a while the reverse will 

 take place, Avhen the last will be fully expanded and the first fidly contracted. 

 But in these alternate movements there is a moment when both halves are in 

 a state of apparent equilibrium, though one be in the process of emptying and 

 the other in the process of filling ; while at the moment an equal amount of liquid 

 has been pressed from that half which is contracting into that half which is filling, 

 the symmetry is most complete. These alternate contractions are nearly as regular 

 as the movements of diastole and systole of the heart, and take place by a constant 

 balancing of the fluid one way and the other. The difficulty of watching this 

 singular circulation arises chiefly from the necessity of keeping the living animal in 

 one and the same position, as the slightest obliquity will interfere with the perspective, 

 so as to make it altogether impossiljle to follow the natural movements ; and unless 

 the parts are placed in a strictly identical position, those which are in jiairs will 

 create confusion, as they may come into various positions, jDresenting apparently a 

 close connection with parts to which they are not at all related. Again, the peripheric 

 tubes extending in vertical arches over the 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 circum- 

 stance, that the arrangement of parts, owing to the bilateral symmetry of the body, 

 appears entirely different when viewed in profile, from the side, or in front, and 

 it will be plain, that, unless the observer keep in mind several 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 impossiljle for him to trace 

 for a moment its structure while alive ; and he will only have constantly before 

 his eyes the tantahziug image of a piece of machinery, appareirtly very compli- 

 cated, the structure of which he has to decipher while it is moving, but moving 

 almost too fixst to allow liuu to seize the connection of the difierent parts as they 

 pass along, and which is not only deranged, but destroyed, the moment it is stopped. 

 It was under such circumstances 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 ^^firticle 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 plam to me, that, even after 

 being colored, the circidating fluid continued its normal course, I must say that 



