1342 



CRUSTACEA. 



other current, which we shall soon mention ; a second (B) passes a 

 little forward and outward, gives off blood to the first pair of feet, 

 continues outward, accompanies the muscles of the mandible, and 

 runs down the body near its margin ; a third (C) goes forward out- 

 side of the base of the second pair of antennae, continues to the ante- 

 rior antennae, to which it gives a portion of its blood, turns inward, 

 passing into the frontal cephalic segment, and along its articulation to 

 the medial line. At this place the currents, meeting from the two 

 sides, flow down the medial line to the mouth. 



The second instance of valvular action occurs in this last medial 

 current, between the second joints of the second pair of antennae (fig. 

 1 a). There is a single valve, composed of a membrane, playing back- 

 ward 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 neces- 

 sary for the flowing of the blood. Indeed, the blood flows in by the 

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

 presume that the depression of this organ is produced by the muscular 

 band which has been described as passing across the posterior part of 

 the mouth, to an attachment in the shell on each side (fig. 1^?). 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 cephalothoracic 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 remain- 

 ing portion below flows to the base of each of the natatory legs and 



