44 COMMISSION OF CONSERVATION 



At 20 the general shade is distinctly reddish brown, which becomes deeper 

 with advancing growth. This tint is characteristic of the oyster 

 larva, and of itself can serve to distinguish the older stages from every 

 other bivalve larva. At first I supposed it had to do with the red sand- 

 stone rocks, that in Prince Edward Island form such pleasing contrast 

 with the green herbage above them, and influence the apparent shade of 

 the water and its reflections, the mirage, the beach, the great sand-dunes, 

 the roads and even the people, who are sometimes nicknamed "Redfeet." 

 But the oyster larvse of Caraquet and other places along the New Bruns- 

 wick coast present the same reddish-brown colour, although red sand-stone 

 rocks are absent and larvse of mussels, clams, quahaugs and the rest are 

 not red, even at Malpeque. 



In the larger larva? the colour becomes so deep as to interfere with 

 the transparency, but wherever soft parts, as the mantle, are protruded 

 beyond the edge of the shell they are of a distinct pink. 



Velum. — When mounted on a slide and covered with a cover-slip, 

 living larvffi are generally quiet, with all parts withdrawn into the closed 

 shell. If the cover-slip is supported so as to accommodate a greater 

 depth of water, however, a few of them will soon show signs of life. To 

 encourage this, fresh sea-water of the same temperature as they are 

 accustomed to should be used, and for low powers of the microscope an 

 open watch-glass may be conveniently employed, instead of the slide and 

 cover-slip. 



Of the parts that can be protruded beyond the bounds of the shell, 

 the most conspicuous is the velum. At first the larva is covered with 

 small cilia, except in the region of the shell-gland. The broader anterior 

 end develops a crown of longer, thicker, more powerful cilia, the ciliated 

 disk or prototroch (Plate I, figs. 9, 10, 11), which in the trochophore, as 

 well as in the youngest shell-bearing stages of the veliger, is a stiff, fixed 

 organ, incapable of lateral movements, folding or retraction. But, parallel 

 with the growth of the shell, there are muscle-fibres differentiated, which 

 can now find a solid place of insertion. Some of these extend from the 

 prototroch to the region of the umbos and soon become effective in with- 

 drawing the swimming organ back between the shell-valves. The changes 

 in growth of the shell, which result in broadening its posterior end and in 

 carrying the umbos backwards, are no doubt correlated with the require- 

 ments for room to retract the prototroch and to give a distant insertion 

 to the retractor fibres. As the viscera become crowded backwards the 

 prototroch grows larger, and acquires a degree of freedom from the rest of 

 the body by a constriction between the two. This permits free expansion 

 of the margins and increases the surface and swimming power of the organ 

 of locomotion, which can arch over the anterior end of the shell somewhat 

 like a veil, hence the name 'velum' (Plate I, figs. 12, 13, 14, 15, 19, 20). 

 In being withdrawn into the shell it folds up the middle (Plate Y, fig. 30), 



