June 2, 1911] 



SCIENCE 



863 



that without doubt all biologists, myself in- 

 cluded, will take great pleasure and satisfac- 

 tion in welcoming to this field all careful 

 workers, whose services should be much appre- 

 ciated where there is such a vast amount of 

 labor waiting to be performed. 



Charles H. T. Townsejjd 



PlUEA, PEEtj, 



January 29, 1911 



SPECIAL AETICLES 



METAMORPHOSIS WITHOUT PARASITISM IN THE 



UNIONID^ 



It has been known for a long time that in 

 the genus Strophitus Eafinesque the embryos 

 and giochidia are embedded in short cylindri- 

 cal cords which are composed of a semi-trans- 

 lucent gelatinous substance, and that these 

 cords, which are closely packed together, like 

 chalk crayons in a box, lie transversely in the 

 water-tubes of the marsupium. The blunt 

 ends of the cords are seen through the thin 

 lamella of the outer gill, which in this genus, 

 as in Anodonta and others, constitutes the 

 marsupium. The position of the masses of 

 embryos, while contained within the gill, is so 

 unusual that Simpson in his " Synopsis of the 

 Naiades " established a special group, the 

 Diagenffi, for Strophitus — the only genus of 

 the family in which this peculiarity exists. 

 In other genera the embryos are conglutinated 

 more or less closely to form flat plates or 

 cylindrical masses, each one of which is con- 

 tained in a separate water-tube and lies ver- 

 tically in the marsupium. 



So far as we are aware, Isaac Lea^ was the 

 first to observe this interesting arrangement 

 which he described and figured, rather crudely 

 to be sure, in Strophitus undulatus {Anodonta 

 undulata). In several subsequent communi- 

 cations' he added further details and illustra- 

 tions, and also mentioned the occurrence of the 

 transversely placed cords, or " sacks " as he 

 called them, in S. edentulus. He recorded the 

 former species as being gravid from Septem- 

 ber until March, and described the extrusion 



^ ' ' Observations on the Genus Unio, ' ' Vol. II., 

 1838. 

 'Ibid., Vols. VI., X., 1858, 1863. 



of the cords from the female, as well as the 

 remarkable emergence of the giochidia from 

 the interior of the cords after the latter have 

 been discharged. " The sacks were discharged 

 into the water by the parent," he says, " from 

 day to day, for about a month in the middle of 

 winter. Eight or ten young were generally in 

 each sack, but some were so short as only to 

 have room for one or two. . . . Immediately 

 when the sacks came out from between the 

 valves of the parent, most of the young were 

 seen to be attached by the dorsal margin to 

 the outer portion of the sack, as if it were a 

 placenta." 



The essential points in these observations 

 have since been verified by other investigators. 

 Sterki,^ following the suggestion of Lea, has 

 called the cords, which differ strikingly from 

 the conglutinated masses of Uiiio and other 

 genera, " placentae " — thus indicating that he 

 considered them to have a nutritive function. 

 He also described the extrusion of the gio- 

 chidia, when placed in water, and their at- 

 tachment to the cord " by a short byssus 

 thread whose proximal end is attached to the 

 soft parts of the young." He further states 

 that the giochidia are enclosed in the pla- 

 eentse when the latter are first discharged, and 

 that after their extrusion they remain at- 

 tached for some time. 



Ortmann,' in a paper on the breeding sea- 

 sons of the ITnionidse of Pennsylvania, says of 

 S. undulatus, which he regards as identical 

 with edentulus: 



I found this species gravid in the months of 

 July, August, September, October; also in May. 

 The latest date is May 22, 1908 (one out of eleven 

 individuals). Among numerous specimens col- 

 lected on May 14 and May 27, 1908, no gravid 

 females were present, and during the month of 

 June such were never found, although a good 

 number of specimens were collected. The earliest 

 date again is July 11. This gives an "interim" 

 from the end of May to about the middle of July. 



In a later paper Ortmann'' states that the 

 discharge of the cords, which he proposes to 



" Nautilus, Vol. XII., 1898. 

 *Ibid., Vol. XXII., 1909. 

 'Ibid., Vol. XXIII., 1910. 



