308 ROBERT TRACY JACKSON ON THE 



The mantle borders of PI. xxrv, fig. 4, are free throughout their extent, with the excep- 

 tion of a small area ncai- the umbos, where the borders of the two folds are confluent. 

 The margins are thickly set with tentacles which are slightly pigmented with black. 

 When undisturbed and quietly feeding, the mantle stretches out to the margin of the shell 

 on allsides and even protrudes considerably beyond it. It moves actively, being constantly 

 retracted and extended by the radial muscles shown in my figure and described by 

 Ryder (G3). The radial muscles are best seen from the left side when the spat is 

 viewed through the glass to which attached, as the lower valve is thinner and less pig- 

 mented than is the npper right valve. The marginal tentacles ai-e constantly in motion, 

 being continually elongated and retracted. When elongated they are used with a tenta- 

 tive, feeler-like motion, and if ii-iitated are capable of extreme i)rolongation. The gills 

 and palps move frequently and suddenly. The adductoi- muscle in PI. xxiv, fig. 4, is 

 seen to be composed of two parts, as is that of the adult, PI. xxv, fig. 12, and a condition 

 of the adductor similar to that of Ostrea is seen in Perna. 



During later stages of growth the soft parts rapidly assume the form, structure and 

 position which they occupy in the adult, PI. xxv, fig. 12. The month parts ai-e revolved 

 towards the umbos and the anus and adductor are revolved in the opposite direction, 

 towards the free ends of the valves. The gills increase in complexity by the develop- 

 ment of cross partitions within the gill chambers, between the contiguous faces of the 

 direct and reflected portions of each gill lamella. These partitions form walls joining 

 gill filaments and separated from succeeding walls by eight or ten filaments not so joined. 

 Their existence gives rise to the folds characteristic of the adult oyster's gill. Large 

 water tubes are thus formed, each tube bordered by eight or ten gill filaments on either 

 side with their network of cross bars. 



An epitome will now be given of the changes by which the adductor muscles of the 

 young oyster become transformed into those of the adult and the relation of these changes 

 to what exists in othei- adult Pelecyjiods is discussed. The embryo first passes through 

 an early single-muscled stage, fig. 25, p. 300, in which the single adductor is the equiv- 

 alent of the anterior adductor of dimyarians. This stage, as stated I believe, is a typi- 

 cal early stage of all Pelecypod embryos.' Later, theoj^ster has two adductors, PI. xxiv, 

 figs. 1-2, entitling it to ancestral kinship with dimyarian Pelecypoda. Finally the ante- 

 rior adductor is lost, and the remaining postei'ior muscle is alone retained. 



Li the embryonic oyster, fig. 23, p. 299, the mouth and anus are closely approximated 

 on the ventral aspect of the body. Passing to figs. 24 and 25, pp. 299-300, the anterior 

 adductor muscle has developed, and the anus has revolved dorsally. In PI. xxiv, figs. 

 1-2, the posterior adductor has developed on the ventral side of the intestine, which 

 wi-aps around that organ as in later stages, and in all adult Pelecypods. In the last fig- 

 m-es the velum, interposed between the anterior adductor muscle and the mouth, prevents 

 the mouth revolving dorsally as it otherwise might have done. Passing to PI. xxrv, 

 fig. 3, the mouth has i-evolved dorsally similarly to the anu.^, and lies nearer the umbos 

 than in previous stages. Howevei-, as noted by Ryder (63) in his studies, it still opens 

 downward and not as directly forwai-d as in the adult. In PI. xxiv, fig. 4, the mouth 

 lies still nearer to the umbos of the valves and from this stage to the adult, PI. xxv, 



' Vide p. 280, and section xv. 



