HIGH SCHOOL ZOOLOGY. 231 



oped by the heart. This organ is " systemic," driving the blood, 

 which it has collected from the gills, forward and backward 

 throughout the body. The most spacious part of the ccelom is 

 the pericardium surrounding the heart ; it communicates with 

 the outside (the mantle-cavity) by means of a pair of nephridia 

 (the Organ of Bojanus), which open into the pericardium ante- 

 riorly, and then turn ujjon themselves in such a way, that the 

 distal non-glandular part of each tube lies above the proximal 

 glandular part, and opens in nearly the same plane as the peri- 

 cardial opening. 



There are two gill-plates ; each is formed of a number of vei'- 

 tical filaments attached to the side of the body- wall (us may be 

 seen to the right of * in Fig. 155) and curved upon themselves, 

 the inner series to the inside, the outer to the outside. The 

 plate results from the union of filaments to their neighbours 

 in front and behind ; it is double, owing to the recurving of 

 each filament, but the two layers are separated above, although 

 the space between them is partly obliterated by junctions be- 

 low. 



"Water is sucked through the inhalent or branchial siphon by 

 means of ciliated cells on the gill-plates ; the current sets through 

 the surface of the plates into the spaces between the two layers, 

 whence it is swept out through the exhalent or cloacal siphon, 

 carrying with it the excreta from the kidneys and intestine. 

 Solid particles contained in the water are swept forwards 

 towards the mouth, and guided into it by ciliated cells on the 

 tentacles. 



The reproductive organs are situated in the foot, and the eggs 

 undergo the greater part of their development in the inter- 

 lamellar space of the outer gills, which are thus turned into 

 brood-pouches. The distribution of the larvse (Glochidia) is 

 provided for by their escaping thence and fastening themselves 



