358 COMPARATIVE AX ATOMY chap. 



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



In the Crustacea (and in the Arthropoda generally) we 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 lacunas 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 with the blood sinus 

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

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

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

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

 ward through the lacunar system more or less constantly in contact 

 with the integument of the body and limbs where respiration takes 

 place, and finally re-enters the pericardial sinus. 



Entomostraca. — The scheme just sketched corresponds more 

 exactly with the circulatory system of the Brandiiopoda {Phyllopoda) 

 than with that of any other known Crustacean. The contractile 

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

 the whole 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 without ostia which 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 pericardial sinus by 

 a septum stretched transversely over the enteric wall. The respiration 

 takes place in the whole 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 always much shortened, sac or pouch- 

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

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

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

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

 ostia through which the blood flows into the heart are generally pro- 



