No. 26.] ARTHROSTRACA OF CONNECTICUT. 29 



to a single pair corresponding in position to the last one of 

 Other forms. The arteries are reduced to an anterior and a 

 posterior aorta, the openings into which from the heart are 

 guarded by valves opening outward. The posterior aorta lies 

 close against the intestine ; the anterior one breaks up principally 

 about the nervous matter of the head. The principal lacunae are 

 the pericardium and the ventral lacuna; they have no walls of 

 their own but are simply the spaces between other organs through 

 which the blood courses somewhat irregularly. 



Internally the Amphipoda and Isopoda differ most strikingly 

 as regards the circulatory system. In the latter group the heart 

 lies principally in the abdomen and ends blindly behind. Besides 

 the anterior aorta, like that of the Amphipoda, there are five 

 pairs of arteries extending forwards. On the ventral side of 

 the body is a large blood sinus which is paired in the thorax but 

 single in the abdomen. Five pairs of veins convey blood from 

 the pleopods, which are respiratory in function in the Isopoda, 

 to the pericardial lacuna. In the Isopoda the arterial system is 

 much more complete than in the Amphipoda, where the lacunae 

 are relatively more extensive. There is accordingly a more [ler- 

 fect separation of arterial and venous blood in the Isopoda. 



The rapidity of the pulsations of the heart is considerable. Ir 

 young specimens of Microdeutopus Delia Valle found the rate 

 to be two hundred per minute. 



Closely associated with the difference in the position of the 

 heart in the Amphipoda and Isopoda is that of the respiratory 

 organs. In the former group the gills have the form of flattened 

 oval sacs, depending from the upper posterior corner of the 

 coxal plates, internal to the principal axis of the limbs. (Figs. 3 

 and 26.) The gills are confined to the last six thoracic append- 

 ages. In a few genera the last pair of pereiopods also lack gills. 



In Ampelisca and Corophium the second gnathopods, of the 

 females only, are devoid of these appendages and in Ericthonius 

 and Cerapus this gill is absent in both sexes. The gills are ex- 

 ceedingly large in Gammarus, but in Orchestia they are very 

 much reduced, in accordance with the semiterrestrial habit of 

 the genus, and are twisted on their long axes instead of being flat 

 plates parallel to the coxal plates. The anterior gills are usually 

 larger than the posterior ones. 



