522 



ZOOLOGY 



SECT. 



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 

 renewal of the water in their neighbourhood. The renal organ 

 or shell-gland (Fig. 433) consists of a coiled urinary tube (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. 434) 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 ossophageal con- 

 nectives pass back- 

 wards and downwards 

 to join the ventral 

 nerve-cord, which con- 

 sists of a double chain 

 of ganglia (gn. 1-4) 

 united by longitudinal 

 connectives and trans- 

 verse 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 



FIG. 433. Shell-gland of Apus, diagrammatic, or. ring round the gullet, 

 cephalic artery ; b. bladder ; h. heart ; m. second maxilla ; wr11<vn in frrmt infr & 

 t.i I. end-sac; uc. urinary tube. (From Bernard.) 



small visceral ganglion 



(v. gn.). Passing backwards the nerve-chain diminishes in size, and 

 comes to an end at about the level of the last pair of abdominal 

 feet (Fig. 431). 



The origin of the nerves given off from the central nervous 

 system presents many points of interest. From the fourth ganglion 

 of the ventral cord backwards each pair of appendages has its own 

 pair of ganglia, the metameric correspondence between the limbs 

 and the nervous system being complete. The mandibles and the 

 first maxillae also receive nerves, each from their own pair of ganglia, 

 their serial homology with the more typical appendages being 

 thus confirmed. But the second maxillae receive their nerves 

 (mx. 2) from the connectives between the third and fourth ganglia : 



