iU 



apparatvis is present in the newly- attached larvae, though 

 here a secretion is thrown out for the attachment of the 

 left valve, and does not form a byssus tliread. In forms 

 like Teredo and Ostrea tlie byssus serves for the attach- 

 ment of the young bivalves, and apparently it has the same 

 purpose in othei- forms in v/hich it is present in t!-.e young 

 (Pecten) , but is lost in the adult. In Sphaeriiim it serves 

 to attach the viviparous larva to the wall of the brood 

 chamber. All the known facts go to show that the byssus- 

 apparatus has been developed to assist in the transforma- 

 tion of the free-swimming palagic larva into the bivalve 

 viith an attached or other settled mode of life; and that, 

 the transformation having taken place, the bysus may be 

 lost;or may be retained in forms which are permanently at- 

 tached, but lack other means than the byssus for attach- 

 ment. 



The sheath of the crystalline style is well develop- 

 ed in the newly-attached larva. However, everything in- 

 dicated its fonmtion from the posterior end of the stom- 

 ach. If we imagine the intestine leaving the blind end 

 of the sheath, we get a fom very like Nucula and Yoldia, 

 in which the posterior half of the stomach has the same 



