156 BULLETIN OK THE BUREAU OF FISHERIES. 



inhibited contractions, and when the glochidia were allowed to remain in them they 

 finally died in the expanded condition. When the Mg salts, however, were used in 

 stronger solutions, closure of the valves occurred after a few spasmodic contractions. 



IV. THE PARASITISM. 



ARTIFICIAL INFECTION OF FISH. 



In anv investigation which attempts to ascertain the facts of most importance for 

 the artificial propagation of a species, attention is at once directed to those points in the 

 life history where wholesale destruction of the individuals is most likely to occur. These 

 points of wholesale waste are usually to be found in the earlier part of the individual's 

 existence rather than during its adult life and are often preventable by artificial means. 

 In common with other animals which must overcome the chances of parasitism, the 

 Unionidse produce enormous numbers of eggs, the great majority of which are by virtue 

 of the brooding habit of the female mussel carried safely through their embryonic period 

 and discharged as glochidia. We have not attempted to estimate the numbers of 

 glochidia carried by full-grown adult females, but anyone who has seen them taken 

 from the gills knows that they must be numbered by the hundreds of thousands, or even 

 millions, and had these glochidia any great chance of survival and development to the 

 adult stage the supply of mussels would far exceed anything which has ever been known 

 in nature. When, however, the next stage of the larval history is sought for in nature, 

 it becomes apparent that we have reached a point in the life cycle where the destruction 

 and waste of individuals is wholesale and probably in excess of that which occurs at any 

 other stage. There is no evidence, save in the case of the species Strophitus edentulus, 

 the metamorphosis of which we have discussed under another heading of this paper, 

 that any one of the Unionidse can pass beyond the glochidial stage without becoming a 

 parasite upon some fish, for the failure of glochidia to develop when left in water has 

 been observed by all investigators since Leeuwenhoek. 



The large element of chance involved in this shift from parent to fish, which has 

 already been emphasized in our discussion of the glochidium, is again apparent when 

 fish are examined in nature with a view to determining the abundance of the parasitic 

 larvae under the conditions of natural infection, for all investigators agree that the para- 

 sites exist in numbers which are insignificant when compared with the masses of glochidia 

 which occur in the parent mussels. Only an occasional fish is found to be infected and 

 it thus becomes clear that the purely accidental nature of the infection makes necessary 

 the production of glochidia in such abundance as to overcome by sheer force of numbers 

 the chances of destruction. Fish become infected in nature by occasional glochidia, 

 but the chance that any fish will carry under natural conditions the number of glochidia 

 which our experiments have shown that individual fish are capable of carrying, when 

 artificially infected, is a negligible quantity. Here, then, we have the point of greatest 

 destruction in the life cycle of the Unionidse; and the point of attack for artificial propa- 

 gation is clear. The fish must be made to carry more glochidia. Under experimental 



