MORPHOLOGY OP LAMELLIBRANCHIATE MOLLUSKS. 405 



In a majority of cases the sexes are separate, but hermaphroditism exists in very- 

 many forms. Pelseneer points out the fact that it exists in isolated forms, species in 

 certain genera, as in Ostrea, Pecten, and Gardiuni, and whole genera in certain families, 

 as in the case of Gyclus, Pisidlum, and Entovalva. He says : " Chez tons ces animaux la 

 glande genitale eUc-meme est hermaphrodite; elleproduit dans toute son 6tendue des 

 ceufs et des spermatozoides, ou bien si une partie est specialisee pour donner naissance a 

 chacun de ces produits, ceux-ci sont amenes au dehors par une seul canal, hermaphro- 

 dite {Pecten).'''' 



The European oyster, Ostrea edulis, is hermaphrodite, but in the American form, 

 0. virginiana, the sexes are separate. While rearing the young of this form from the 

 eggs at Woods Holl, with Mr. Harrison of the Johns Hopkins University, we found a 

 specimen apparently containing both eggs and spermatozoa. On sectioning parts of 

 the generative gland, I found it to be hermaphrodite, as was suspected. The large 

 follicles (Fig. 72, PI. lxxxix) were generally more or less united, and their lumens 

 everywhere, in specimens taken from a number of different parts of the glands, ap- 

 peared full of ripe spermatozoa. The ova, with their distinct nuclei, were apparently 

 unripe, though many were free from the follicle walls. The majority of them were 

 yet attached. The cells of the follicular epithelium showed ova in all stages of growth, 

 and none of them, apparently, were giving rise to spermatozoa. 



This specimen was obtained late in June, near the end of the breeding season. 

 Whether or not its sexual glands indicate the change in function from male to female 

 or from female to male, which possibility has been suggested for some lamellibranchs, 

 I can not say; but, as abnormal hermaphroditism often occurs in all groups of animals, 

 this may be an example of it. 



The sexual gland of Pecten irradians is hermaphrodite, and there are an ovary and 

 a testis on each side of the visceral mass. Both glands are ventral to the liver (Fig. 

 44, g) and have a spongy appearance. The testis is the more ventral of the two and 

 is of a cream color. The ovary, situated above this, has a reddish hue, which is very 

 marked in the living animal. 



Fig. 71 represents a section passing vertically through the outer wall of the vis- 

 ceral mass, where the testis and ovary are closely apposed. The body wall is repre- 

 sented at ep and consists of a single layer of columnar, ciliated, epithelium cells, 

 whose nuclei are about equally distant from thefr outer ends and the thick basement 

 membrane {hm). In this epithelium are many conspicuous gland cells {glc). Be- 

 tween it and the follicles of the generative gland is a thick layer of connective 

 tissue, extending in between the follicles. The follicles of the ovary [ov) are not so 

 regular in outline when seen in section as those of the testis {t). The walls of the 

 latter bear a follicular epithelium {fep). In the ovary, the cells of this layer are in 

 all stages of development into eggs. The eggs themselves, crowding the follicles, 

 possess a very thick egg membrane and their protoplasm is finely granular. A duct 

 from the follicles is seen at d. 



The mother cells of the spermatozoa {fep) are circular and of constant size in the 

 follicles of the testis {t). As we follow the mass of cells inward from these mother 

 cells they become very gradually smaller and smaller, until their final duisions result 

 in the spermatozoa. These are so arranged that their "tails," in forming, project in 

 extended masses toward the lumen of the follicle and give it a radiating appearance. 



