BRACHIOPODA. 



579 



The ganglia and commissure are in 



which are developed a variable number of ganglia. The most 

 general arrangement appears to be one small supra-oesophageal and 

 one larger suboesophageal swelling. The siiboesophageal gives off 

 nerves to the dorsal mantle-lobe, the arms, and adductor muscles, 

 and to two small ganglia which supply the ventral mantle-lobe and 

 the peduncular muscles. 

 immediate contact with 

 the ectoderm. Special 

 organs of sense have 

 not been described. 



The vascular sys- 

 tem has a canalicular 

 character. There is a 

 heart placed on the 

 dorsal side of the 

 stomach in the dorsal 

 mesentery. It gives 

 off a vessel which 

 passes forwards along 

 the oesophagus in the 

 dorsal mesentery ; this 

 divides into two, which 

 are continued into the 

 arms as the tentacular 

 vessels, and are con- 

 nected with a vessel 

 which surrounds the 

 oesophagus. In addi- 

 tion to the above there 



are two pairs of lateral FlG 463 _view of mantle-chamber of Rhynchonella psittaeea, 



^ e ventral mantle lobe turned back (after Hancock). 

 a dorsal, & ventral mantle lobe ; c, c mantle sinuses ; d, d 

 genital glands; g circumpallial vessel; j occlusor muscles; 

 ^ vent; ral nephridia opening at I ; m arms ; n lip of brachial 

 groove ; o position of mouth concealed by lip ; p tentacles 

 contracted ; q oesophagus. 



vessels runnill " fl'Olll 

 tlie neart to tlie gen- 



Organs. 



The blood is Colour- 



less. 



Our knowledge of the vascular system and its ramifications is 

 very small, as may be gathered from the statement made by some 

 observers that the blood vessels communicate with the coelom. This 

 is a statement which has been made of other groups, and has 

 always been disproved with the progress of knowledge ; and in the 

 present instance, having regard to the difficulties of the observations 



