OF THE ACALEPHyE OF NORTH AMERICA. 321 



as transverse. But here then right and left would remain doubtful. For the reason 

 above alluded to, I have, therefore, no hesitation in tracing my comparisons through all 

 the classes of Radiata, and introduce here the Braehiopods only that they may be 

 remembered in this connection. The parts already mentioned in a general way are not 

 the only ones which have reference to the bilateral arrangement. The tentacles arise in 

 two sacs, extending inwards in a vertical direction towards the opposite side of the body, 

 and reaching a point about as far below the higher centre of the animal as the point from 

 which the tentacles issue is from the lower centre. These sacs are connected with another 

 very large sac, giving out two wide hollow forks in opposite directions in the transverse 

 diameter of the body, which branch at right angles parallel to the longitudinal diameter, 

 and again give out each a branch at right angles, that is to say, parallel to the transverse 

 diameter, thus forming the eight forks of eight hollow tubes, following for their whole 

 length the inner surfaces of the eigiit vertical rows of locomotive fringes. The position 

 of this complicated system, therefore, presents a regular symmetry, two main trunks 

 penetrating symmetrically right and left from the central cavity, and branching in such 

 a manner as to reach on each side with four arms the four vertical rows of locomo- 

 tive fringes, and also to supply the sac from which the tentacles arise. This cavity 

 is full of liquid, which is in constant movement by the agency of vibratory cilia, but 

 also under the influence of a regular pulsation of the system in the two halves of the 

 body, which alternate in their contraction and dilatation ; so that at one time the 

 fluid moves, to a considerable extent, from one side to the other, and next returns by 

 the contraction of the opposite side through the same tube in the opposite direction, 

 presenting something similar to what exists in Salpa, under circumstances differing 

 considerably, however, as will be shown presently. The point to which I now wish 

 especially to invite attention is the circumstance, that there is a large central cavity 

 branching in a very symmetrical way in the right and left parts of the body, and un- 

 dergoing a rhythmic movement of contraction and dilatation, alternating between the 

 two sides. This cavity, which I shall call the chymiferous cavity, is not to be mis- 

 taken for the digestive cavity, which constitutes a sac within it hanging downwards 

 from the mouth for about two thirds of the length of the animal, but which com- 

 municates with it in the same manner as the digestive cavity of Actinia communicates 

 with the main cavity of the body. This chymiferous cavity finally has two apertures, 

 by which it communicates with the surrounding water, and through which it dis- 

 charges the refuse chyme. These apertures are placed in a symmetrica! position on 

 the two sides of the area opposite the mouth near its centre, obliquely opposite each 

 other, so that one is in the anterior half upon one side of the body, the other in 



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