Gills. 



Each gill is attached by one lamella to a muscular membrane that 

 serves to elevate the gills when the shell is closed. The other lamella 

 is not attached. This arrangement makes it possible for the water to 

 be thrown from the shell in swimming without injuring the gills. The 

 inter-filamenter junctions are composed of cilia near the margins of the 

 gills, and of tissue near the suspensory membranes. Their blood vascu- 

 lar supply is intricate. (See pp. 21-30, and figs. II, 12, 17, 18, 19, 20, 

 and 21.) 

 Muscular System. 



The anterior adductor muscle is lost at an early period of develop- 

 ment. The posterior adductor muscle is distinctly separated into an 

 anterior and a posterior portion. The anterior portion, which is much 

 the larger of the two, may be cut without causing the snell valves to 

 gap. If the posterior portion is cut without injuring the anterior por- 

 tion, the valves immediately open. 



Muscles for withdrawing the margins of the mantle and the gills are 

 well developed. Only the left retractor muscle of the foot is present 

 in the adult animal. (See pp. 31 and 32, and figs. 10, 16, 19, 20, and 26.) 



Excretory Organs. - 



These are essentially rather large sacs with glandular walls. They 

 receive the genital ducts near their pericardial ends. (See pp. 32 and 

 33, and figs. 13 and 20.) 



Genital Organs. 



The genital organs are large, pink in the female, and white in the 

 male. The genital ducts join the excretory organs near their pericardial 

 ends. (See pp. 33-35-) 



Circulatory System. 



The large size of the animal makes it possible to inject the vascular 

 system successfully. Blood from the mantle is returned immediately to 

 the heart. Most of the blood from other portions is returned to the kid- 

 neys, from which it is carried to the gills and then back to the 'heart. 

 A portion may dodge the kidneys and go to the gills. Blood seems to 

 act both as blood and lymph. (See pp. 35-42, and figs. 9, n, 13, 14, 17, 

 and 21.) 



Nervous System. 



The cerebral and pedal ganglia are small and somewhat removed from 

 their usual positions. The visceral ganglia are very large and compli- 



62 



