534 



ZOOLOGY 



prfivided with valves opening inwards, and is continued in front 

 into a narrow tube, tire cqihalic 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 h.-Kmogiobin, and contains amoeboid corpuscles. 

 As already mentioned, the function of respiration is discharged 

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

 pliefl with blood and the movements of which ensure a constant 



Fig. 42.[j. — Transverse section of Apus. cm. rauscles to feet ; di>. dorso-ventral museles ; e. eggs ; 

 dm. dorsal muscles ; if. ovary ; dv, dorso-venti'al muscles ; h. heart ; i. Intestine ; »i. 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. 4^6) consists of a coiled urinary tuie {uc.) 

 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 (b.) 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 (esophageal 

 connectives pass backwards and downwards to join the ventral nerve- 

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

 by longitvidinal 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 (y. gn.). Passing backwards the nerve-chain 



