534 



ZOOLOGY 



SECT. 



provided with valves opening inwards, and is continued in front 

 into a narrow tube, the cephalic artery (c. art.), which extends into 

 the head and gives off near its origin a pair of arteries to the shell- 

 glands (Fig. 422). When the heart contracts, the blood is driven 

 through these arteries to the head and carapace : it then travels 

 backwards in the intestinal sinus, passes to the limbs, and is 

 returned to the pericardial sinus, finally re-entering the heart, 

 during its diastole, through the ostia. The plasma of the blood 

 is coloured red by haemoglobin, and contains amoeboid corpuscles. 

 As already mentioned, the function of respiration is discharged 

 by the flabella and bracts of the feet, which are abundantly sup- 

 plied with blood and the movements of which ensure a constant 



-•m 



Fig. 425. — Transverse section of Apus. em. muscles to feet ; dv. dorso-ventral muscles ; e. eggs ; 

 dm. dorsal muscles ; g. ovary ; dv. dorso-veutral muscles ; h. heart ; i. intestine ; m. partition 

 between intestinal and lateral sinus ; vm. ventral muscles. (From Bernard.) 



renewal of the water in their neighbourhood. The renal organ 



or shell-gland (Fig. 426) consists of a coiled urinary tube (nc.) 

 lying between the two layers of the carapace and lined by gland- 

 cells. At one end the tube is connected with an end-sac (ts.), 

 also lined with glandular epithelium ; at the other it dilates into 

 a small bladder (6.) which opens on the second maxilla (m.). 



The nervous system (Fig. 427) is constructed on the annulate 

 type. There is a squarish brain {br.) situated in the dorsal region 

 of the head, beneath the eyes. From it a pair of aisophageal 

 connectives pass backwards and downwards to join the ventral nerve- 

 cord, which consists of a double chain of ganglia (gn. 1-4) united 

 by longitudinal connectives and transverse commissures so as to 

 have a ladder-like appearance. The first pair of ganglia lies 

 immediately behind the mouth, and sends off visceral nerves which 

 join to form a ring round the gullet, swollen in front into a small 

 visceral ganglion (v. gn.). Passing backwards the nerve-chain 



