226 MANUAL OF ZOOLOGY SECT. 



and therefore takes the only other course open to it, viz. into 

 the arteries. When the heart relaxes, the blood in the 

 arteries is prevented from regurgitating by the valves at their 

 origins, and the pressure of blood in the pericardial sinus 

 forces open the valves of the ostia and so fills the heart. 

 Thus, in virtue of the successive contractions of the heart, 

 and of the disposition of the valves, the blood is kept 

 constantly moving in one direction viz., from the heart by 

 the arteries to the various organs of the body, where it 

 receives carbonic acid and other waste matters ; thence by 

 sinuses into the great sternal sinus ; from the sternal sinus 

 by afferent branchial veins to the gills, where it exchanges 

 carbonic acid for oxygen ; from the gills by efferent 

 branchial veins to the branchiocardiac veins, thence into the 

 pericardial sinus, and so to the heart once more. 



The nervous system (Fig. 120) consists of a brain (g.} and 

 a ventral nerve-cord, united by oesophageal connectives (sc.). 

 The ventral cord is double, but the right and left halves 

 have undergone partial fusion, so that the ganglia, and in 

 the abdomen the connectives also, appear single instead of 

 double. The ventral cord contains twelve of these ganglia, 

 the first is infra-cesophageal and is larger than the others, 

 being formed by the union of the ganglia belonging to the 

 last three cephalic and first three thoracic segments. All 

 the remaining segments have their own ganglia, with the 

 exception of the telson, which is supplied from the ganglion 

 of the preceding segment. There is a visceral system of 

 nerves (s) supplying the stomach, originating in part from 

 the brain and in part from the cesophageal connectives. 



The eyes differ entirely in structure from those of any 

 animal that has been described hitherto. Each is a com- 

 pound structure, being made up of a large number of 

 distinct elements termed the ommatidea. The chitinous 



