V CRUSTACEA— BLOOD-VASCULAR SYSTEM 359 



vided with valves to prevent its return into the pericardial cavity on 

 the contraction of the heart. 



The heart is always placed above the intestine in the most anterior 

 trunk region. 



With regard to the presence of the heart in the Entomostraca, all 

 Cladocera possess hearts. Among the Ostracoda it is only found in 

 the Halocypridm and Cypidinidce, among the Copepoda only in the 

 Calanidce, Pontellida, and Branchiura. In the latter it lies far back in 

 front of the so-called caudal fin and is continued anteriorly in a long 

 aorta as far as the brain. In the Cirripedia a separate blood-vascular 

 system is altogether wanting. 



It would be a mistake to assume that those Entomostraca which appear to be 

 simple forms because of the want of a heart and generally of a separate blood- 

 vascular system, are therefore also original forms. As in the worms, so also in Crus- 

 taceans, the want of this system of organs must certainly be considered as a derived 

 condition. The causes why a reduction of the heaH goes as far as complete dis- 

 appearance are indeed only known to a very small extent. Small size of body may 

 here and there have some influence, occasionally the rhythmical movements of other 

 inner organs {e.g. the stomach of many Copepoda) seem to suffice to set in circulation 

 the blood or coelomic fluid in the lacunar system ; a heart is thus rendered superfluous. 



"We have one remarkable exception to all that has been said above as to the 

 circulatory system in the Crustacea. One genus of parasitic Copepoda (Lernan- 

 thropus) possesses a richly branched blood-vascular system widely spread in the body 

 and its appendages, and completely closed from the body cavity. A heart is wanting. 

 The yellowish red blood is propelled along the principal vessels by the movements 

 of the enteric canal, and flows forwards in two ventral longitudinal trunks and back- 

 wards through an unpaired dorsal vessel. 



Malaeostraea, Leptostraea. — A knowledge of the circulation in 

 the Leptostraea {Nebalia, Fig. 196, p. 293) is of great importance; 

 it recalls in many respects that of the Branchiopoda, and in others 

 points to that of the Malaeostraea. The long tubular heart stretches 

 from the most posterior head region through the whole thorax 

 into the 4th abdominal segment, and possesses 7 pairs of ostia. The 

 3 most anterior of these lie laterally in the heart in the posterior 

 part of the cephalic region, the 3 following dorsally in the 2d, 4th, 

 and 5th thoracic segments, and the 7th and largest in the 6th thoracic 

 segment. In the last 2 thoracic segments, and in the abdomen the 

 heart has no ostia. These pairs of (venous) ostia through which the 

 blood enters the heart from the pericardial sinus, are, as in all Crusta- 

 ceans, provided with valves. The heart is continued into an anterior 

 and a posterior aorta, through which the blood flows out into 

 the body. Valves hinder its return from the two aorta into the 

 heart. Besides the aorta, branched arteries occur in both pairs of 

 antennae and in the abdomen. The principal portions of the lacunar 

 blood -vascular system are the pericardial sinus and a sinus lying 

 under the intestine. 



The respiration is in all cases specially active at the thin inner 



