PHYLOGENY OF THE PELECYPODA. 



299 



stomach which coinmiuiicates witli the outside through the intermediatiou of an in- 

 testinal canal. 



Our fig. 23 is distinctly the essential equivalent of Brooks' figs. 38-42. Our fig. 24 

 again is reasonably close to Brooks' fig. 44. At this last stage there are two well-devel- 

 oped valves, a stomach, mouth and anus, a ciliated foot and a large velum. In fig. 24 a 



Fig. 23. 



Fig. 24. 



DEVELOPMENT OF OSTREA EDULIS (AFTEK I10R.ST). 



Fig. 23. — Kmlirvn at more advanced stage than lig. 22, FiG. 24. — Einbr.vo still more developed; v, velum; oe, 



side view, with shell more developed. oesophagus; st, stouiach; a, anus; and kp, cephalic plate. 



body is rejjresented which, although Horst does not mention it, appears to be the be- 

 ginning of the adductor muscle of the valves. It is in the same position as that occu- 

 pied by the adductor in Ilorst's fig. 16, Brooks' fig. 44 and our fig. 25, p. 300. 



I have descriljed as the phylembryo an early period of development (see section iii) 

 where, however, the class characters are recognizable, and I consider figs. 24-25 as phyl- 

 embryonic stages of the oyster. They have two equal valves and an anterior adductor 

 muscle, characters which render the embryo refei-able to the phylum or class Pelecypoda, 

 to which it pi'operly belong's, although not referable to any known adult form. 



The month and anus when developed arose in close pro.Kimity on the ventral border 

 of the embryo, Brooks' fig. 38, our fig. 23, which is a generalized characteristic of a mol- 

 luscau embi-yo. It is seen in fig. 24 that the first adductor muscle develops anteriorly 

 and close to the dorsal margin of the velum. The retractor muscles of the velum and 

 liver then develop. The anus, from its early ventral position, revolves dorsally in the 

 plane of the valves (see Brooks' figs. 38-44). Tlie intestine by interstitial growth 

 makes a single loop-like curve on the left side; this with some other slight changes 

 brings us to the stage, fig. 25, described by Huxley (26). Ryder (63), remarking on 

 this stage, notes that the intestine is already flexed on itself in much the same manner 

 as in the adult, though it does not extend as far anteriorly, and its position is modified 

 later by flexions of the stomach and oesophagus. 



The shell of the stage, fig. 25, is symmeti-ical, equivalvular, and of about the period of 

 development of PI. xxiv, figs. 13-16, of the developing .shell of this same species. The 

 mouth has yet no palps, a sac-like process of the stomach on either side represents 

 the liver, two pairs of muscles serve to retract the vehiin, and the single adductor muscle 

 closes the valves. Huxley ob.serves that this early anterior adductor cannot be the 

 equivalent of the single adductor of the adult as it is on the ventral side of the alimen- 

 tary canal whereas that of the adult is on the dorsal side. He says that a second posterior 



