214 CTENOPIIORiE. Part II. 



locomotive llapi)ors, two other ))i"anches the sacs from which the tentacles issue, 

 and two others l'ollo-\viiig the walls of the digestive cavity, — the fom- latter arising 

 from the main lateral trunks in the trend of the transverse diameter, wliile the 

 forks which su])pl_y the locomotive flappers trend at first in the direction of the 

 longitudinal diameter, and emit each another fork parallel to the transverse di;ime- 

 ter, so that all jiarts 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 the order which 

 will assist us in the understanding of all the other systems, rather than to follow 

 a physiological principle. 



Though the form is appjirently well determined and regular, even superficial 

 investigation will satisiy the observer that it is constantly changing within more 

 extensive limits than might he at first suspected. In the first place, the apparently 

 spherical form is not only frequently altered into an ovate by the vertical 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 completely shut, it disappears almost entirely ; and its position 

 is scarcely marked liy any thing more than an indistinct outline, towards which 

 the actinal ends of the rows of locomotive flappers converge. When half-way open 

 or while opening, it assumes an oval form, like a fissure, across the body, which 

 l;)ecomes gradually more and more elongated, then widens, and finally expands into 

 an ample, circular, funnel-shaped depression. These movements are rather slow, and 

 may be compared to the undulations of a slug or snail adapting its mouth to the 

 form of its food. The changes in Pleurobrachia, however, do not seem to be 

 called forth by the approach of food, l)ut are rather the result of a natural dispo- 

 sition in this animal to Ije in an attitude ready to seize upon its prey. Various 

 aspects of the mouth are represented in my former paper. 



The whole bulk of the body of Pleurobrachia, excepting the spaces occupied by 

 the digestive and the chj'miferous system and the tentacular sockets, is a solid mass 

 of closely packed cells, most of which are of enormous size (PL IP. Fi//. 2i). Such 

 is the extreme transparency of these cells, that it is very difficidt to follow their 

 contour except in profile, and on this account the thickness of their walls has been 

 mistaken for long and slender muscular fibres ; and this illusion is oftentimes 

 heightened when the wall wrinkles {F/(/. 24, I/, c) during contraction, and appears 

 like shrunken fibres. But there is no muscular systenr apart from the constituent 

 cells of the body, and therefore no contractile filjres of any kind so grouped as 



