Chap. III. GENUS PLEUROBRACHIA. 213 



cavity ; for the main trunks of the chymiferous tubes, with their forks branching 

 in a very symmetrical way in the right and left parts of the body, imdergo a 

 rhythmic movement of contraction and dilatation, alternating Ijctweeu the two sides. 

 In those Ctenophoras which have an oral chymiferous tube communicating with the 

 recurrent branches of the ambulacral tubes, the bilateral circulation is not so promi- 

 nent. The axial cavity, which I have called funnel, is not to be mistaken for 

 the digestive cavity, though it extends in the same axis with it, in the direction 

 of the abactinal pole ; nor is the digestive cavity entirely surrounded hy the 

 chymiferous system, as I had supposed when I published my paper in the Memoirs 

 of the American Academy, but only flanked on each side by so wide a coeliac 

 tube as readily to appear like a sac surrounding the whole digestive cavity.^ The 

 funnel has two apertures, by which it communicates with the surrounding water, 

 and through which it discharges the refuse chyme. These apertures are placed 

 in a symmetrical position on the two sides of the oblong area, ojiposite the mouth 

 and near its centre, obliquely opposite each other; so that one is in the anterior 

 half upon one side of the body, the other in the posterior half upon the other 

 side. These openings are generally shut; but they open at intervals to discharge 

 the fecal matters, and are afterwards instantaneously shut again. It is very diffi- 

 cult to catch these movements ; and even after I had seen them open and shut, 

 I have frequently watched for days without observing a repetition of the oper- 

 ation, which I have, however, seen so many times now, that I entertain no doubt 

 resj)ecting the position of these openings and their natural function. Moreover, 

 balls of fecal matters may almost constantly be seen floating with a rotating motion 

 below these apertures. 



This sketch gives as yet but a slight, very incomplete, and superficial idea of 

 the remarkable complication of structure which may be observed in these animals; 

 but such a preliminary illustration was necessary before undertaking a minute 

 description of all parts and their natural relations. And, before alluding to these 

 details, I would request the reader to bear the following points in mind: that 

 Pleurobrachia is not sti'ictly spherical, nor even strictly circular, in any direction; 

 that there is a longitudinal axis, which pagses through the mouth and the area 

 opposite ; that the tentacles are in the transverse axis, at right angles Avith the 

 fissure of the mouth ; that the digestive cavity trends in the vertical and in the 

 longitudinal axis ; that the chymiferous system branches symmetrically in the right 

 and left halves of the body, eight Ijranches reaching the eight vertical rows of 



^ Milne-EJw.ii-ds has made the same mistake in Sc. Nat. 4e ser. vol. 7, PI. XVI. Fig. 2. Tlie coeliac 

 representing the chymiferous cavity as surrounding tubes occupy only the more deeply colored part of 

 the sac into which the month leads. Comp. Ann. his drawing, ou the sides of the digestive cavity. 



