of the VentriculitUe of the Chalk. 361 



likely to offer some difficulty in the way of the free circulation 

 of the sea-water. Confident however that, if my interpretation 

 of the nature of these animals were correct, the means of free 

 circulation must exist and might be discovered, I resumed the 

 examination with an increased series of specimens. The result 

 was the strengthening of all conclusions as to the physiology 

 of the whole family in general, and of this species in particular, 

 by the discovery of a contrivance by which this end was perfectly 

 effected; a contrivance which, for its novelty and simplicity, may 

 well claim the inquirer's attention. 



Equidistant, or nearly so, along each lateral margin, I found 

 what at first I took for a larger form of depression, but which, on 

 dissection, I found to be actual perforations* through the mem- 

 brane. These are found present from near the root to the ex- 

 treme angular expansion already named. A comparison of many 

 specimens satisfied me that these perforations are never absent ; 

 that they are always ranged in the same way, and that their size 

 is proportioned to that of the entire animal and to the width be- 

 tween the walls of the arms. They are usually circular, some- 

 times oval. They vary from half a line to two lines in longest 

 diameter, seldom however attaining this last size. 



It became at once evident that this simple provision could 

 have but one end ; but that that end it would fully effect. That 

 end was the fulfilling of the very purpose whose incomplete- 

 ness had before been felt. A constant access and circulation 

 of sea-water would be maintained over all the inner surfaces 

 of these lobes, deep and narrow as they are ; the water, admitted 

 at the opening of the great central cavity, coursing out, by 

 the continual action of numberless ciliated tentacles, through 

 these marginal perforations. And herein it is that these perfo- 

 rations differ from those in the group Aperti, and do not bring 

 this species within that group. The large central opening was 

 amply sufficient, in this case, for the admission of sea-water, but 

 the peculiar form of the arms interposed difficulties in the way 

 of its free change and circulation, which is equally necessary to 

 the well-being of these creatures. Hence this beautifully simple 

 contrivance for the water, admitted by the central orifice, to pass 

 out through these perforations. In the group Aperti, on the 

 other hand, each perforated lobe offered the principal, if not in 

 every case the only, means of both access and circulation of the 



* In some of Michelin's figures traces of some of these are found, which, 

 when their nature is known, will be recognized, but which, as they appear 

 in those figures, suggest nothing but irregular depressions : they attracted, 

 indeed, no attention of that author, but, in his descriptions, they are wholly 

 unnoticed, the margins being only said to be, — in the same words as the 

 " superficiebus laminum," — " pprforalis," which, as applied, is erroneous. 



