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ART. III. — Descriptions of the Embryonic Forms of Thirty-eight species of Unionid^e. 



By Isaac Lea, LL. D. 



Among the numerous observers of the anatomy and habits of the family Unionidce, 

 very few have given any attention to their ovulation and incubation. Poli, Carus 

 and Pfeiffer have given descriptions and figures of the ova and young of a few 

 European species, but they have not observed the embryonic differences of the 

 species, and it does not seem to have occurred to them, that there might be an 

 essential difference in the various species in this important stage of existence. 



I have occasionally, in some of my papers, described and figured the forms and 

 condition of the branchial uterus* of a number of species, but until within the last 

 three years, I have not entered into the close examination of their embryonic state, 

 and particularly at the period of the developement of the embryo when it is matured 

 in the ovum, but still retained in the ovisacks of the branchial uterusf of the gravid 

 parent. 



*On a consultation with my friend, Dr. Leidy, we concluded, in order to avoid a confusion of names, 

 and to make the terminology more specific, to adopt this name for those parts which have heretofore been 

 called, we think erroneously, the oviducts, as the oviduct really forms only the means of passage of the mature 

 ova to the organ, probably after it has received fecundation, ovulation having taken place in the ovarium. 



fProf. Agassiz, Archiv fiir Naturgeschichte, 1852, page 44, in referring to my figures of the branchial uterus 

 (so-called oviducts,) in several species of the Unionidce, (Trans. Am. Phil. Soc. New. Ser., vol. 6, pi. 15,*) states 

 that " the ovisacks (eiersacken) give a bad idea, because the connection of these sacks (which he calls oviducts) 

 with the gills is entirely overlooked, and in the figure itself, which is intended to represent the entire animal, 

 there are no gills present." 



Now all the above criticisms are totally incorrect. The specimens were carefully prepared by me to show 

 expressly the different forms of the so-called oviducts and their connection with the branchiae in several of our 

 Pennsylvania species. My artist, Mr. Drayton, who was subsequently artist to the Exploring Expedition 

 under Capt. Wilkes, had no superior in this country, as the plates of the great work illustrating the Natural 

 History of that voyage fully prove. In the four species of which the so-called oviducts are represented, they 

 are distinct and well characterized, and I have stated in the text that " the oviduct will be found to be placed 

 in the posterior portion of the branchiae." Thus they are not only carefully figured on the plate, but they are 

 expressly stated in the text, to be placed in the branchiae, and yet Prof. Agassiz asserts that I have entirely 

 overlooked this connection ! Itwas not intended on my part, as asserted by Prof. Agassiz, to give " the whole 

 animal ;" the plate was made chiefly to give the form and connection of " oviducts and branchiae." The mantle 

 in each case was pushed back or cutoff, to enable the artist to give the different forms of the oviducts perfectly, 

 and their connection with the branchiae, and they are so given. There was no great care taken to elucidate 

 in this plate any other organs of these animals ; the four principal drawings were made for the purpose of 

 exhibiting the organs which he says I have " entirely overlooked," and the excellent artist succeeded admirably 

 in representing them, and they do not " give a bad idea of the ovisack" (so-called oviducts) of the four species, 

 but on the contrary a very correct idea, as any one at a glance can perceive. 



* Observations on the Genus Unio, &c, vol. 2. 

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