4o6 



ANIMAL LIFE-HISTORIES 



Fig. 927. — Veliger T,nrva (12 days old) of an 

 Edible Mussel [Mytihis edulis), x 240 



v., Velum; v.ni.y one of the muscles for 

 pulling back velum; _/, foot; sh., shell; s. in., 

 shell muscle; ??/., mouth; st., stomach; i., 

 intestine; d.g., digestive gland. 



breathing and feeding of the adult form (see vol. ii, p. 398). 

 The abundant supply of oxygen which developing eggs require is 

 thus provided for. In the Common Oyster {Ostrea edulis) the 

 eggs hatch out in the space between mantle-flaps and gills; in 



Freshwater Mussels [Unio and Anodon) 

 the outer gill -plates serve as brood- 

 pouches; while in some other fresh- 

 water forms (Cyrena and Cyclas) the 

 inner gill-plates discharge this office. 



The egg of marine bivalves, and 

 more rarely that of freshwater forms, 

 hatches out into a free-swimming larva, 

 which is usually a Veliger (fig. 927). 

 This may be regarded as a modified 

 Trochosphere, for it possesses the cir- 

 clets of cilia, thickenino- of the skin to 

 form a brain, and some of the other 

 features which characterize a larva of 

 that kind (see p. 359). But the swimming arrangements are here 

 more efficient, for the base of the head lobe is drawn out into a 

 flap or velum, upon the edge of which the front circlet of cilia is 

 placed. And the velum can be drawn back by special muscle- 

 bands so as to get a purchase upon the water, thus propelling 

 the larva forwards. Slower movements are 

 executed by means of the cilia alone. The 

 fully developed Veliger is obviously a Mol- 

 lusc, for it has grown a foot and secreted a 

 ?hell. 



An unusual and aberrant life-history char- 

 acterizes most of the Freshwater Mussels 

 (species of Unio and Anodon). The eggs 

 hatch out in the outer gill-plates as an extra- 

 ordinary larva known as a Glochidium (fig. 

 928), the name having been given at a time when it was thought 

 to be an independent species. It possesses a pair of triangular 

 shells with strongly hooked tips, a long trailing sticky thread 

 (byssus), and several peculiar pointed projections believed to be 

 special sense-organs. There is some difference of opinion as to 

 what happens when the Glochidia are ejected from the gills of 

 the parent. It is generally stated that they swim about actively 



Sh 



Fig, 928. — Glochidium of Fresh- 

 water Mussel [Anodonta), en- 

 larged- .M, Shell-muscle; s, special 

 sense-organs; sh., shell; F, byssus 

 thread [cut short). 



