TNVERTEBRATA. 



313 



forced along the arteries. This " ostiate " condition of the 

 heart is characteristic of the class Arthropoda : we may 

 imagine its origin by supposing the enlargement of the 

 main veins entering the heart until they come in contact 

 with one another all around the heart. Then let their 

 party-walls be absorbed, and an ostiate heart surrounded 

 by a blood- sinus is the result. On this view the six ostia 

 represent six veins entering the heart, and the last remnant 



Fig. 157. DIAGRAMMATIC TRANSVERSE SECTION OF THORAX OF ABTAOUS. 

 The arrows show the course of tho blood. (After Hnxlcy.) 



of their walls is seen in the strands of connective tissue 

 which connect the heart to the sides of the sinus. 



17. Arteries and Sinuses. The arteries leaving the 

 heart (fig. 176) are: 



(1) A median ophthalmic artery running forward to 

 supply the front part of the gizzard, and adjoining regions. 



(2) A pair of antennary arteries, supplying not only the 

 antenn?e, but almost the whole anterior portion of the body. 



(3) A pair of small hepatic arteries, to the digestive 

 gland. 



