OF THK ACALEPH^ OF NORTH AMERICA. 339 



macliiiierv, in wliich hydrostatic power, elastic levers, and muscular action give rise to 

 highly complicated combinations and most diversified phenomena. 



In the first place, the cavity itself from which each of the two tentacles issues (Plate 

 IV. Fig. 1, 2, and 5) is a wide, elongated, pear-shaped sac, the rounded extremity of 

 which is turned upwards and bent obliquely outwards, so that its convexity is turned 

 towards the centre, and its blind sac upwards and outwards, and its ojjcn lower ex- 

 tremity downwards and outwards. In this cavity, to which ti)e surrounding water 

 has free access through the lower opening, the tentacle, with its complicated base, 

 is attached by a broader surface to the inner side of its upper part. And though the 

 central cavity of the body communicates freely, through the vertical tubes of the main 

 horizontal trunk, with the base and curved hook of the tentacular apparatus, there is 

 no free passage from one of the cavities into the other. The fluid which is injected 

 into the vertical tubes runs back through the same channels into the main trunk, and the 

 water which washes the central cavity of the tentacular ajjparatus empties through the 

 same lower opening by which it is introduced. In a state of dilatation water penetrates 

 from without into the pear-shaped sac, and chyle is injected from within into the vertical 

 tubes ; and in a state of contraction the chymiferous tubes are emptied at the same 

 time that the water is pressed out. (Plate IV. Fig. 1, 2, and 5.) During these alternate 

 contractions and dilatations, the tentacle itself may be coiled up in the cavity, or drawn 

 out at full length, though in the most dilated state the threads generally hang out. But 

 there seems to be also an antagonism, in a middle state of dilatation, between the filling 

 of the vertical chymiferous tubes and the protrusion of the tentacle, the motions of which 

 depend partly upon the muscular action of the apparatus to which it is attached, and 

 partly upon the state of dilatation of the tubes on the inner surface of its base. The 

 dilatation and contraction of the tentacular cavities depend upon the contraction and 

 dilatation of the vertical bundles of muscular fibres of the two lateral zones, and more 

 especially of the sphincters around their lower extremities ; the sphincter reducing the 

 diameter of the cavity, and shutting its opening, while the vertical bundle shortens and 

 widens the whole cavity. 



To form a correct idea of the ever-changing state of these parts, it is necessary to 

 keep in mind their form and structure, as well as their relative position. The base from 

 which the tentacle arises (Plate IV. Fig. 5) is an oblong disk encircled by elastic 

 springs around its sides, which are bent inwards at its upper end, and turned outwards 

 and downwards in the form of a hook projecting outside. The flat base of this disk 

 forms the inner partition of the cavity in its upper part, and the vertical chymiferous 

 tubes, which penetrate into the base of the tentacular apparatus, rise close together 



