322 CONTRIBUTIONS TO THE NATURAL HISTORY 



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 difficult to catch these movements, and even after I had seen 

 them open and shut, I have frequently watched days for them without observing a 

 repetition of the operation, which I have, however, seen so many times now, that 

 I entertain no doubt respecting the position of these openings, and their natural func- 

 tion. Moreover, balls of fecal matters will 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 

 strictly spherical, nor even strictly circular, in its somewhat elongated form ; that there 

 is a longitudinal axis, which passes through the mouth and the area opposite ; that 

 the tentacles are in the longer axis, at right angles with the fissure of the mouth ; that 

 the digestive cavity hangs in a large circulatory cavity branching symmetrically in the 

 right and left halves of the body, the branches, eight in number, reaching the eight 

 vertical rows of locomotive fringes, two other branches providing the sacs from 

 which the tentacles issue, and two others following the walls of the digestive cavity, 

 these four latter rising from the main lateral stems of the central cavity along the 

 transverse diameter, the forks supplying the locomotive fringes, on the contrary, branch- 

 ing first parallel to the longitudinal diameter, and emitting each another fork parallel to 

 the transverse diameter ; so that all parts have a precise geometrical relation to each 

 other; and finally, that the right half of this system alternates in its contractions 

 with the left half. 



In the special investigation of the minute structure of the different systems of organs 

 developed in these animals, it will be better to proceed in such an order as will assist us 

 in the understanding of all the other systems, rather than upon a physiological principle. 



Though the form is apparendy well determined and regular, even superficial in- 

 vestigation will satisfy the observer that it is constantly changing within more ex- 

 tensive limits than the appearances would lead him to suspect. In the first place, the 

 apparently spherical form is not only frequently altered into an ovate by the elongation 

 of the mass, but it even assumes, at times, a form rather cylindrical than ovate, especially 

 on the side of the mouth, by the extensive dilatation of this opening. The changes which 

 the mouth assumes in its outlines are very extensive and frequent. When shut, and 



