Septembee 4, 1914] 



SCIENCE 



355 



Lave a nutritive function. The absence of a 

 placenta or any matrix about the glochidia of 

 Anodonta imbecillis is of interest since the 

 non-existence of parasitism in this case is 

 apparently under quite different conditions 

 from those governing in Strophitus. I have 

 mentioned above the extreme lightness of the 

 juvenile shells in Anodonta imbecillis up to a 

 considerable size. In the resulting buoyancy 

 we have undoubtedly a device for distribution 

 of the young and thus a compensatory provi- 

 sion for the loss of the usual means of dis- 

 tribution by fishes. 



At the U. S. Fisheries Station, Fairport, 

 Iowa, there are several ponds used for retain- 

 ing fish seined from the Mississippi River. In 

 these ponds have been found a great many 

 young mussels of species known to be para- 

 sitic on fish and evidently introduced into the 

 ponds during the parasitic stage. A concrete 

 reservoir was at first used to supply the water 

 to the ponds. Upon examining the bottom 

 of this reservoir in 1912 the presence of 

 mussels (Unionidse) was discovered. This at 

 first seemed surprising as no fish had been put 

 in the reservoir, but it was noteworthy that 

 these mussels were all of one species, Anodonta 

 imbecillis. The explanation given for their 

 presence was that owing to the lightness of 

 their shells in the juvenile stage they had 

 been pumped through the intake pipe from the 

 river. This explanation made without the 

 knowledge of the non-parasitic metamorphosis 

 was undoubtedly the correct one and I give 

 the incident only as an illustration of the 

 possibilities of their distribution in water 

 currents. It is my opinion that the so-called 

 " placenta " of Strophitus edentulus has a 

 similar distributing function; the cords being 

 buoyant may be readily carried by flowing 

 water. In this ease, however, the mechanism 

 is quite different and thus we have in the two 

 species different devices for accomplishing the 

 same purpose. 



The question arises as to the nutrition of 

 these non-parasitic glochidia during the period 

 of metamorphosis. Both of these species un- 

 doubtedly have come from parasitic ancestors 



which received at this stage nutriment from 

 their hosts so that one would look for some 

 provision for nutrition here. 



I have not as yet observed any such pro- 

 vision in Anodonta imbecillis and I do not 

 Imow that this has been demonstrated for 

 Strophitus. In the latter case to prove a 

 nutritive function for the cords it would seem, 

 necessary to demonstrate an absorption of the 

 substance of the cords by the young mussels. 

 As the cords swell considerably upon leaving 

 the gills such a determination is difficult. 



The discovery of so fundamental a change 

 of habit, apparently derived independently by 

 two lines, should give opportunity for many 

 interesting comparisons; for Anodonta im- 

 becillis already possessing the distinction of 

 being an hermaphroditic species it adds an- 

 other eccentricity to its reputation.^" 



Arthur D. Howard 



U. S. Biological Laboratokt, 

 Faikport, Iowa 



laboratory notes 

 i. embedding trays 

 In the laboratories of this coimtry and 

 Europe a variety of receptacles are used to 

 hold the melted paraffin in embedding. Doubt- 

 less all of them have certain advantages and 

 it is certain that most of them have annoying 

 disadvantages. Paper trays are not stiff 

 enough for large cakes and are very likely to 

 stick. L-shaped bars of metal that can be 

 adjusted to a variety of sizes are placed on 

 glass plates. They are very likely to leak if 

 the paraffine must be kept liquid any length of 



10 Since the above was written I have been able 

 to secure infections and encystment on fishes with 

 Anodonta imiecillis as well as StropMtus edentulus. 

 In the latter complete metamorphosis was observed. 

 Thus for edentulus we have indicated facultative 

 parasitism while in the other we have a persistence 

 of the parasitic reaction at least when artificially 

 brought in contact with a host. Metamorphosis on 

 fishes was not secured in A. imbecillis. Abundant 

 additional evidence is at hand that development in 

 this {imbecillis) species normally proceeds without 



