VASCULAE SYSTEM OF AKTHKOPODA. 



285 



ov 



respiratory organs. Before the blood from them gets to the heart, 

 it passes into the pericardial sinus. 



In the other Arachnida the many-chambered cardiac tube is 

 reduced, and re- 

 sembles in cha- ° ^^ ^f _*' 6_ 



racter that of the 

 Insecta. It always 

 lies in the abdo- 

 men; inthcAranea 

 and Opilionida it 

 is provided with 

 three pairs of ostia, 

 by which it is di- 

 vided into cham- 

 bers. From the 

 most anterior of 

 these an artery is 

 continued into the 

 cephalothorax : in 

 Lycosa this is di- 

 vided into two 

 trunks (Fig. 148), 

 each of which gives 

 off' branches for the 

 eyes and appen- 

 dages. The hin- 

 dermost chamber 

 opens at the end 



of the abdomen, and the blood- current from it corresponds to that 

 which is distributed by the caudal artery in the Scorpionea. Since | 

 there is no pericardial sinus the blood passes to the respiratory 

 organs, and from them to the heart, by lacunar passages only. 



Among the Pycnogonida this apparatus is limited to a three- 

 chambered heart, into which two pairs of ostia open ; in the Acarina 

 no heai't at all seems to be developed. 



Fig. 118. Circulatory organs of Lycosa. A Seen from 

 above ; B From the side, o Eyes. 1,2,3,4,5,6 Appendages. 

 P Lungs. C Heart, or Venous ostia of the heart. The 

 arrows indicate the direction of the blood (after Clapaivde). 



Excretory Organs. 



§ 221. 



The apparatus, which in the Vermes is formed of looped canals, 

 is found in a modified form in the Crustacea. One of the organs, 

 which represent it, consists of a coiled tube placed below the integu- 

 ment of the head, and opening at the base of the second (outer) 

 pair of antennas. In the Entomostraca, this organ is confined to the 

 larva, in which it has been made out in most divisions. It may 

 perhaps be retained in the Cirripedia as the so-called " cement 

 glands/' which lie in the stalk of the Lepadidas, and open at its 



