196 BULLETIN OF THE BUREAU OF FISHERIES. 



much larger than the posterior adductor muscle, but as development proceeds it becomes 

 narrower and finally much smaller than the posterior adductor muscle. 



The foot, which is a muscular glandular organ, appears next as a hollow outgrowth 

 of ectodermal cells into which there grows a large mass of mesodermal tissue from 

 immediately behind the velum (fig. 185, Ft, p. 197). At this stage of development speci- 

 mens are 0.36 mm. long and show three or four gill papillae. In its early appearance 

 the foot is wedge-shaped, but as growth continues it becomes long, slender, and highly 

 contractile, and is covered with fine cilia (figs. 187 and 188, Ft, p. 198). During this 

 development a deep invagination occurs on the posterior ventral side of the foot, which 

 results in the formation of the byssus gland (fig. 188, ByT). 



As soon as the foot begins to take on form the posterior retractor muscle of the 

 foot and byssus can be seen running from the base of the foot back over the posterior 

 adductor muscle to the shell where it is inserted (figs. 185 and 186, PRet). At this 

 stage the young mussel has attained a length of 0.385 mm. 



The anterior retractor muscle of the foot and byssus is the last one developed. 

 The smallest specimen in which it was observed was 0.512 mm. long (fig. 187, ARet). 



GILLS. 



The development of the gills was worked out first by Tacaze-Duthiers (1856) and 

 more recently by Rice (190S). According to these investigators a papilla arises on 

 each side of the body between the mantle and median visceral mass when the larva is 

 about 0.3 mm. long, or approximately at the stage shown in figure 184. New papilke 

 arise behind these in succession and grow downward to form the branchial filaments 

 (figs. 185-188, BrF). The free ends of the filaments are thickened, and as they increase 

 in number the anterior and posterior faces of the succeeding swollen tips fuse together 

 making a continuous membrane of the sheet of filaments. As the filaments continue 

 to grow they are reflected inward to form the ascending lamellar leaf. At the same 

 time interfilamentar junctions are developed by the interlocking of some specially long 

 cilia which hold the adjacent filaments together firmly. Immediately following this 

 stage, in fact before the ascending lamella is well formed, there arises, just outside and 

 parallel with the first series, a similar row of papillas which grow downward to form the 

 filaments of the descending lamella of the outer gill. As growth continues they bend 

 outward and are reflected upward to produce the ascending lamella of the outer gill. 

 The outer branchial filaments begin to appear when the mussel is about 1.4 mm. long 

 and possesses 20 of the inner gill filaments. In specimens that had reached a length 

 of 1.6 mm. Rice (1908) found 25 filaments in the inner gill and 15 in the outer one. 



KIDNEY. 



The first sign of the kidney is a mass of small mesodermal cells immediately in 

 front of the posterior adductor muscle in specimens 0.36 mm. long (fig. 185, K, p. 197). 

 The anterior end of the mass grows forward, finally forming a pair of longitudinal canals 

 which lie on either side of the body at the roots of the gills (fig. 188, K). 



NERVOUS SYSTEM. 



The ganglionic centers arise independently from thickenings of the ectoderm and 

 become connected later by commissural fibers which grow out from them. Wilson 



