58 COMPARATIVE ANATOMY. 



of branchiae (Tetrabranchiata), as in Nautilus, receive two 

 branchial veins ; those having but a single pair (Dibranchi- 

 ata), as in Sepia (cuttlefish), receive but one. At the base 

 of each branchia is a muscular chamber, termed branchial 

 heart, which transmits venous blood to the branchiae. The 

 return blood passes through a systemic auricle before enter- 

 ing the ventricle, thence to be distributed through the body. 

 Currents of water over gill surfaces induced by muscular 

 action in mantle. 



ARTICULATA. Eotifera and Entozoa. (See water-vascular 

 system, p. 65.) 



Annelida. Blood-vessel system complete. Vessels nu- 

 merous. Many arrangements obtain with different genera, 

 the general plan conforming to that seen in all the higher 

 articulates, in the presence of longitudinal vessels extending 

 the whole length of body. As a rule, a dorsal and ventral 

 canal are seen, occasionally combined with lateral vessels, 

 which may in some instances communicate with a vertical 

 set joining the other two. The position of the respiratory 

 apparatus determines the detail of the blood-vessel system. 

 In Lumbricus (earthworm) there is apparently no apparatus 

 exclusively devoted to respiration. Respiratory organs, how- 

 ever, generally present. These may be arranged in depres- 

 sions upon cephalic segments, as in Nemertes; within the 

 abdominal cavity, as in Hirudo (leech), q. v.; in a brilliantly- 

 colored, spiral tuft to head, as in Serpula; or symmetrically 

 appended to the dorsal surface of each segment, as in Eunice. 



Crustacea. Blood-vessel system incomplete. Heart of a 

 single cavity beneath shell at anterior part of the body. 

 Venous currents empty into a dorsal sinus which encloses 

 the heart. Respiration by branchiae, excepting in low forms, 

 as in Cyclops and Lerncea, where aeration takes place- from 

 the cutaneous surface. Branchiae may be free, as in Lepas 

 (barnacle); or enclosed, as in Homarus (lobster), and com- 

 monly placed at the base of the anterior true or false feet. 

 Constantly, without cilia, currents are secured by muscular 

 action of false feet and appendages. 



Arachnida. Blood-vessel system incomplete. Heart com- 



