258 Description of a Species of Caligus. 



claims to be considered as performing the functions of this organ^ 

 though neither is entitled to that name. One of these systems of 

 valves, the more perfect of the two, is situated in the apex of the 

 posterior thoracic joint, (fig. 6 a, b. ) There are at this place three 

 distinct valves ; two laterally on the back, situated in the dorsal 

 currents which are flowing towards the tail, and one centrally 

 below, giving passage to the ventral current flowing from the 

 tail. The dorsal and ventral valves open alternately. Their ac- 

 tion may be seen in the figures above referred to ; a , represents 

 the dorsal valves as shut, and the ventral open, and b, the dorsal 

 relaxed or open, and the ventral shut. The action of these valves 

 is very regular, and the currents which pass them are more uni- 

 form than those in other parts of the body. The number of pal- 

 pitations has been found to vary from thirty to forty per minute. 



The blood coming down the back* from the head, and also in 

 two lateral currents from the point of intersection of the head, tho- 

 rax and epimeral segments, (fig. 7,) passes the dorsal valves. It 

 continues posteriorly ; a part into the terminal joint of the body, 

 and then up the venter, entering the ventral current at the extrem- 

 ity of the intestine ; another part, into the same ventral current 

 near the centre of the abdomen, and at other varying points. The 

 ventral current passes through the ventral valve under the ante- 

 rior margin of the apron, and continues up the body — washing, 

 at the same time, freely over the intestine and stomach, to the 

 thoracic ganglion, where it divides, and passes each side of this 

 organ. Each of these branches goes ofl" laterally ; one portion 

 (A) enters the adjoining prehensile legs, and returns down the 

 body, uniting with another current which we shall soon mention ; 

 a second (B) passes a little forward and outward, gives off" blood 

 to the third pair of maxillipeds, continues outward, accompanies 

 the muscles of the mandible, and runs down the body near its 

 margin ; a third (C) goes forward outside of the base of the first 

 pair of maxillipeds, continues to the antennae, to which it gives a 

 portion of its blood, turns inward passing into the anterior cephalic 

 segment, and along its articulation to the medial line. At this 

 place the currents meeting from the two sides, flow down the me- 

 dial line to the mouth. 



The course is marked by arrows on figs. 1 and 7. 



