175 



In the marine edible mussel (Mytilus) the filaments are 

 loosely connected by interlocking groups of cilia, ciliary 

 (interfilamentar) junctions, and the edges of their bent- up 

 portions are not fused to the mantle. 



What is the General Course of the Blood Circulation in 

 Anodonta ? 



The impure blood passes from the vena cava (between 

 the kidneys) through the kidney walls to the gills. The 

 pure blood returns from the gills via the auricles to the 

 ventricle which distributes it through two arteries to the 

 body where it flows in irregular channels. The blood 

 which the mantle receives is returned direct to the auricles ; 

 and so the mantle evidently has a respiratory function. 



W hat Functions, other than Respiration, do the Gills dis- 

 charge ? 



1. Nutritive ; certain of the cilia impel food-particles^ 

 towards the mouth. 



2. Reproductive ; the eggs are fertilised and develop 

 between the lamellae of the outer gill ; and the Glochidium 

 larvae are nourished by a secretion from the gills. 



What is the Subsequent History of the Glochidium ? 



When a fish passes near the Mussel, the Glochidia are 

 expelled from the gills. The Glochidium has a " byssus " 

 thread instead of a "foot," and a delicate porous shell 

 with an incurved tooth on each valve. It fastens on to 

 the fins or tail of the fish, becomes embedded under the 

 skin, and there it remains parasitic for some weeks and 

 undergoes metamorphosis. The "byssus" disappears; 

 " foot," gills, and adductors are developed, and the forma- 

 tion of the permanent shell is begun. Then, a miniature 

 adult, it leaves its temporary host^drops to the bottom 

 of the pond or stream. 



