23 



two vessels, one ruuiiing to tlie excretory orgau ou one 

 side, one to that on tlie other. The blood which has 

 hltered through the spongy tissue surrounding the 

 nephridia is collected by the paired vessels which open 

 into the lower branch of the aorta before it forms the 

 perioesophageal ring. There is thus formed a median 

 vertical pericerebral ring in the course of which lie the 

 excretory organs (PL III., fig. 1). 



The paired facial arteries (PL III., fig. 1, fac. art.) 

 arise from the heart at the same level as the anterior 

 aorta, and pass down on either side, ramifying imme- 

 diately below the integument of the head, in the lower 

 part of which they finally lose themselves. The head 

 contains many lacunae filled with blood, which is probably 

 obtained from these arteries, and ultimately finds its way 

 into the ventral blood sinus. 



The sternal or ventral sinus (PL III., fig. 1, st. s.) fills 

 the lower part of the body, below the alimentary canal, 

 and bathes the ventral nerve cord and the two pairs of 

 long hepato-pancreatic diverticula (PL III., fig. 1, h.]).) 

 lying below the alimentary canal. Its cavity is some- 

 what cylindrical, extending from the concave ventral 

 surface of the pericardium to the sternal body-wall; it 

 has apparently no definite walls. It is filled with mixed 

 blood, derived from (1) the two short branches into which 

 the posterior aorta divides, (2) the median anterior aorta, 

 which opens into the sinus after having formed the peri- 

 oesophageal ring, (3) the blood lacunae of the head. In 

 each segment of the thorax and abdomen, it sends off a 

 pair of afferent vessels (PL III., fig. 1, aff. v.) to the 

 appendages of that segment. In each of the first four 

 thoracic and first three abdominal segments, a separate 

 afferent vessel is given off' to each of the large coxal plates 

 (PL III., fig. 1, aff. c. v.), which function as accessory 



