PIIYLOGENY OF THE PELECYPODA. 301 



tlie firmness of the fixation. As the oyster had hcen attached less than twcntj^-four 

 hours, tlie anatomy and shell here rei)resented must have been developed while it was a 

 free, unattached embryo. The fact that the embryonic shell is always fixed by the ex- 

 treme ventral margins only, is further proof that fixation takes place at the close of this 

 period. 



The embryo was yellowish-green and lively, constantly extending and retracting 

 the mantle and gills and moving the palps. The important feature of the two adduc- 

 tor muscles was clearly made out as shown in PI. xxiv, fig. 1, viewed from the right side, 

 and in fig. 2, vicAved from the left side through the transparent glass to which the em- 

 bryo was attached. The intestine wi'aps around the dorsal aspect of the posterior ad- 

 ductor, as in all later stages. A lai-ge, lobed, ciliated velum still exists and is situated , 

 on the ventral aspect of tlie anterior adductor muscle, lying close up to that body. The Z*^ . ))' 

 ciHated and lobed palps are situated close up behind the velum. Club-shaped, tubular, ^ ff^ 

 ciHated gills, PI. xxrv, fig. 1, also fig. G, with the tips not recurved upon themselves, exist. 

 The mantle border is broad and thick, bears no tentacles and is capable of considerable 

 extension beyond the margins of the valves. Within the umbo of the lower left valve 

 was a rhythmically, rapidly pulsating body, as of a large blood-vessel. The liver forms 

 a large, light green mass in the upper part of the body and extending into the lunbos. 



About three and a half hours after the specimen was first seen a slight spat growth 

 had taken place along the ventral margins of the valves. It was kept alive several days, 

 during which growth pi-oceeded rapidly and was studied at intervals; PI. xxiv, fig. 3, 

 being three days older than figs. 1 and 2. We will now discuss this stage, which is of 

 great importance, both in the development of the oyster and related Pelecypoda as well. 



The anterior adductor muscle of the early development, fig. 25, p. 300, is still existent in 

 this stage, PI. xxrv, figs. 1-2, a, ad, but there is another muscle, the posterior adductor, 

 as Will. We have then the interesting feature of a luonomyarian Pelecypod in its early 

 stages possessing two muscles, situated relatively to the alimentary canal as they are in 

 typical adult dimyarians. The anal part of the intestine revolves dorsally during the 

 early development of the oyster and the posterior muscle when formed must have origi- 

 nated on the ventral aspect of that organ. If the posterior muscle had developed on the 

 dorsal side of the intestine it would always have retained that relative position as the 

 permanent anterior adductor of dimyarians does in relation to the mouth and oesophagus. 

 One of the striking characteristics of the ostrean prodissoconch is, that the umbos point 

 posteriorly as shown in PI. xxrv, figs. 1-2. This is contrary to the condition observed 

 in developing dimyarians, as in them the umbos point anteriorly, but an exception is found 

 in N'ucula, the embryo of which, according to Loven, has lunbos directed posteriorly. This 

 character of the umbos is discussed in its serial connections in section xv. 



In Ostrea, fig. 25, indisi)utably the anterior adductor is first developed; also in Car- 

 dium, Modiolaria and Montacuta, PI. xxiii, figs. 9-11, in Pisidium according to Lankester 

 (43) and in Anodon,^ PI. xxrii, figs. 7-8, according to Schierholz^, the anterior ad- 

 ductor is first developed. I have found only two possible exceptions to the rule of the 



' In the embryo of Anodon, PI. xxni, flss. 7-8, the lirst tirst formed adductor is probably In part, at least, the be- 



formed adductor muscle Is in the larger side of the shell. . ginning of the permanent anterior adductor as in the other 



According to Schmidt's figure of a later stage, the larger Pelecypoda al)ove considered, 



•side is the anterior end of the embryo. Therefore the ' Schierholz' paper, see Schmidt's reference list. 



MEMOIRS liOSTON SOC. NAT. HIST., VOL. IV. 41 



