36 OSBORN. [Vol. XVIII. 



beyond the junction of the yolk-duct in Aspidogaster with much 

 thicker walls " lined with high epithelium cells " and surrounded 

 by unicellular glands which contribute the shell substance. I have 

 not been able to find a similar organ in sections of Cotylaspis. 



The eggs are lodged in swollen places in different parts of the 

 uterus beyond the entrance of the yolk-duct. These places are not 

 constant in location, and seem to be formed temporarily to accom- 

 modate the egg. I doubt if the eggs remain stationary in passage, 

 but think it more likely that they are slowly moved along and 

 thrown out of the animal. In some cases I have seen an egg close 

 to the beginning of the uterus and supposed that this .was the place 

 of its formation, but have never seen one in process of formation. 

 The number of the eggs is always small ; there are never more than 

 thirteen present at one time in any of the cases that I have 

 recorded, and generally less. In Aspidogastcr they develop in the 

 uterus to the point where the young worm is moving about within 

 the shell and is ready to emerge (Voeltzkow '88, p. 2^2). In 

 Cotylogaster occidentaUs, according to Nickerson '00, " the eggs 

 when discharged . . . contain a fully formed embryo with 

 unforked intestine and simple subterminal sucker at the posterior 

 end. The body of the embryo is covered in part .with a simple 

 epithelium bearing distinct tufts of cilia." But in Cotylaspis the 

 egg has not undergone the first segmentation by the time it leaves 

 the parent. 



When the mature worms are removed from the mussel and 

 placed in aquaria they generally soon expel a number of eggs, or can 

 be compelled to do so by carefully managed compression. These 

 minute embryos were transferred by means of a capillary pipette 

 under a simple microscope to the center of a slide, where they were 

 covered and submitted to microscopic examination. Fig. 63 is a 

 camera lucida drawing of one of these, and shows its condition as 

 it leaves the parent. Eggs were also seen and studied in situ both 

 in the living parent and in preserved states, both in whole prepara- 

 tions of the parent and in sections. Isolated eggs were kept in 

 aquaria from which they were transferred from time to time to 

 slides for examination. 



The eggs in Cotylaspis are unusually large, measuring in length 

 from .22 mm. to .35 mm. and in width from .10 mm. to .16 mm. 



