358 COMPARATirE AX ATOMY CHAP. 



VII. The Blood-Vascular System and the Body Cavity. 



In the Crustacea (and in the Arthropoda generally) Ave find no 

 closed blood-vascular system. Those portions of the circulatory 

 system which are provided with special walls, stand in open com- 

 munication with blood lacunae. These lacunae have no special walls, 

 but are only spaces between the various organs of the body, and repre- 

 sent the body cavity. 



Scheme of the Circulatory System. According to what is now 

 known of the Crustacea we may imagine the circulatory system in the 

 racial form of these animals to be essentially as follows : A contractile, 

 tubular, dorsal vessel (heart) runs longitudinally through the body 

 in the middle line above the intestine. The direction of the flow of 

 blood in this dorsal vessel is from behind forward, as in the dorsal 

 vessel of the Annulata. In each body segment the dorsal vessel 

 possesses a pair of lateral slit-like apertures, the so-called ostia, through 

 which its interior is in open communication Avith the blood sinus 

 surrounding it ; this is the perieardial sinus, a part of the body cavity. 

 The blood fluid (hremolymph) enters the dorsal vessel through an aper- 

 ture at its posterior end as Avell as through the lateral ostia from the 

 perieardial sinus, and flows out at its anterior end. It then runs back- 

 Avard through the lacunar system more or less constantly in contact 

 Avith the integument of the body and limbs Avhere respiration takes 

 place, and finally re-enters the perieardial sinus. 



Entomostraea.--The scheme just sketched corresponds more 

 exactly Avith the circulatory system of the Branchiqpoda (PhyUopoda) 

 than Avith that of any other knoAvn Crustacean. The contractile 

 dorsal vessel (heart) of Branchipus (Fig. 191, p. 288) runs through 

 the Avhole trunk and possesses a pair of ostia in all segments 

 except the last, in which there is one terminal ostium. Anteriorly 

 the heart is continued into an Aorta Avithout ostia Avhich enters 

 the head and opens into the lacunar system of the body. In 

 this latter system a ventral principal stream from before backward can 

 be distinguished incompletely separated from the perieardial sinus by 

 a septum stretched transversely over the enteric Avail. The respiration 

 takes place in the Avhole surface of the delicate integument of the body 

 and limbs, but is apparently specially active in the branchial sacs. 

 From the ventral principal stream an accessory stream runs into each 

 limb down one side to the point, there to bend round and to run back 

 up the other side to rejoin the principal stream. 



The heart of the other Entomostraca (Figs. 192 and 193, p. 289) 

 (where such an organ occurs) is ahvays much shortened, sac or pouch- 

 shaped and only supplied Avith one pair of ostia. Anteriorly, in front 

 of the anterior terminal ostium, the heart is sometimes (in many 

 Copepoda, Branchiura, and Cladoccrci) continued into a longer or shorter 

 aorta. A posterior ostium is added to the heart of the Copepoda. The 

 ostia through Avhich the blood floAvs into the heart are generally pro- 



