CALIGOIDEA. 1341 



and related species, in which we were unable to detect any blood- 

 corpuscles. 



The circulation in the Caligus is wholly lacunal ; it appears to con- 

 sist of broad irregular streams, passing through the spaces left among 

 the internal organs, and in no part have we discovered distinct vessels. 

 These streams have in general definite directions, yet are seldom 

 uniform, continuous currents. They mostly advance by successive 

 vibrations, depending on the palpitating action of the body. A single 

 centre of circulation, or a heart, this animal can scarcely be said to 

 possess. There are two points in the medial line where there is a 

 valvular action, and each has its 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 posterior thoracic segment (fig. 1 g, g'). There are at this 

 place three distinct valves; two laterally on the back, situated in 

 the dorsal currents which are flowing toivards the tail, and one 

 centrally below, giving passage to the ventral current flowing from 

 the tail. The dorsal and ventral valves open alternately. Their 

 action may be seen in the figures above referred to ; g, represents 

 the dorsal valves as shut, and the ventral open, and g' 9 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 uniform 

 than those in other parts of the body. The number of palpitations 

 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, thorax 

 and epimeral segments (fig. h), passes the dorsal valves. It continues 

 posteriorly \ part, into the terminal joint of the body, and then up 

 the venter, entering the ventral current at the extremity of the intes- 

 tine ; another portion, 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 anterior 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 off laterally ; one portion (which we may call A) enters the ad- 

 joining prehensile legs, and returns down the body, uniting with an- 



* The course is marked by arrows, on figs. 1 a and h. 

 336 



