Description of a Species of Caligns. 259 



The second instance of valvular action occurs in this last medial 

 current, between the second joints of the first pair of maxillipeds, 

 (fig. 1.) There is a single valve, composed of a membrane, 

 playing backward and forward, and thus preventing the return of 

 the blood that has passed it. Between this valve and the mouth 

 there appears to be a large cavity for the reception of the blood, 

 from which it is propelled by a palpitating motion or powerful 

 muscular action in the buccal mass, and surrounding parts. It acts 

 in the following manner : the current enters through the valve 

 while the posterior part of the mouth is elevated ; the valve then 

 closes, and immediately the buccal mass is brought down, and 

 forces it out in a current on each side. This very extraordinary 

 action is carried on uniformly, and is absolutely necessary for the 

 flowing of the blood. Indeed, the blood flows in by the out-cur- 

 rents, until the action of the buccal mass throws it out. We pre- 

 sume that the depression of this organ is produced by the muscu- 

 lar band which has been described as passing across the posterior 

 part of the mouth, to an attachment in the shell on each side, 

 (fig. 12.) If the mouth be cut off, the blood flows out in a large 

 free current, and the animal soon dies from exhaustion. 



A current passes from this cavity each side of the mouth, and 

 others on the back. One portion of the side-current unites with the 

 current C, before described, of which it forms the greater part, and 

 thus soon returns to the buccal cavity. Another portion flows out- 

 ward, following the muscle of the mandible, and unites with B ; 

 this current, thus much enlarged, passes near the margin to the 

 posterior extremity of the cephalo-thoracic segment, returns up by 

 the epimeral articulation, crosses the same just above the junction 

 of the head and thorax, and then turns suddenly backward ; a part 

 flows on the back, forming the lateral current on the back before 

 referred to ; the remaining portion below flows to the base of each 

 of the natatory legs and the apron, and enters them, and at the 

 same time and place, passes in part on the back ; the current 

 from the apron flows laterally down the abdomen. 



Another portion of the side-current leaves the buccal cavity just 

 along side of the mouth, unites with it, and flows to the base of 

 the first pair of natatories. The union of these currents is some- 

 what peculiar : the blood vibrates upward on the venter, to a spot 

 near the base of the prehensile legs, where a portion remains, al- 

 though the main current vibrates back on the venter ; at this mo- 



