﻿332 CONTRIBUTIONS TO THE NATURAL HISTORY 



fibres also show a more or less pennate arrangement (Fig. 1 and 2). But the most par- 

 ticular disposition is in the two lateral bundles (Fig. 2 and 4), which above, towards 

 the mouth, are at first similar to the other vertical bundles, but about the middle of 

 their course diverge to inclose the bases of the tentacles, and form a sort of sphincter 

 to enlarge and shut, in their relaxation and contraction, the opening of the cavity in 

 which the tentacles are contained (Fig. 2, 3, and 4), b being a profile view of those 

 bundles of fibres, e representing their termination upwards, and d their arrangement, 

 when the aperture through which the tentacles issue is fully shut. The peculiar mus- 

 cles which move the partition of the cavity of the tentacles will best be described 

 with that system. 



It has already been mentioned, that there is a wide cavity in the centre of the body 

 of this animal, into which hangs the digestive cavity proper; but the natural relations of 

 these parts are so difficult to appreciate, the ramifications of the cavities so complicated, 

 and nevertheless so regular, and, again, so movable in their constant contractions and 

 dilatations, that, with all the assistance of numerous drawings, as given in Plates III. 

 and IV., I hardly expect to be able to give a correct idea of this apparatus, unless 

 the reader is willing to consider attentively every point of the following description 

 by itself, and to keep, at the same time, constantly in mind the relative connection of 

 all parts, and their bearing upon the general disposition of the body. 



In the first place, let it be remembered, and well understood, that the main cavity 

 undergoes constant changes, as to its size and outlines, according to its temporary state 

 of contraction and dilatation, and that both halves 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 (Plate III. Fig. 2, a) when the other is in the most complete state of con- 

 traction ; and, after a while, the reverse will take place, when b will be fully expanded, 

 and a fully 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 ; but 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 balan- 

 cing of the fluid alternately 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 in order fully to appreciate these movements, as the slightest 

 obliquity will interfere with the perspective in such a manner as to make it altogether 



