38 OSBORN. [Vol. XVIII. 



an exceedingly slow rate .which, however, may be abnormal. They 

 extend to the formation of a morula at the end of three days, and 

 at that time the growth altogether ceased in every instance. My 

 knowledge of the form of the animal after Fig. 78 is an entire 

 blank till the definitive form is reached. The youngest specimens 

 that I have found in the mussel had the outward form of the 

 parent established except with regard to the completion of the 

 ventral sucker, and internally it was like the adult except that no 

 indication of the genitalia was visible. Such specimens are found 

 in the fluids that flow out from the mantle cavity on opening the 

 mussel, and are also found in the mucus that can be scraped from 

 the surface of the foot and the gills in the neighborhood of the 

 kidneys. 



Since the eggs are enclosed in their shell in the upper part of the 

 oviduct it seems clear that they are fertilized before they leave the 

 parent. In that case the standstill to which they come cannot be 

 due to internal causes, and must be due to external ones. 

 Since the egg is supplied with the yolk-cells presumably to 

 serve as food, and is protected from the effect of environ- 

 mental conditions by its shell, it is very difficult to see to 

 what the non-development is due. There is thus a lost interval 

 coming between the early segmentation and the young animal with 

 the chief organs of which we know nothing. The presence of the 

 eyes might lead us to suppose it passes through a period of free 

 life at this point, and its primitiveness among the trematodes would 

 be in harmony with this hypothesis. But how are we to explain 

 the non-development of the eggs on this basis? It is such eggs 

 that ought to be the easiest to rear. If there is not a free stage 

 then we must look for a host, either the mussel itself or an inter- 

 mediate host. There is precedent in the family for the latter from 

 the case of Stichocotyle, Odhner '99 having recently shown that 

 that form has two as had been long surmised. At present however 

 we are not in a position to attempt to fill in the interval by means 

 of any adequate hypothesis. 



Biological Laboratory, Hamline University, 

 March 3, 1900. 



