ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 355 



of the Quadrula group. The eggs pass from the genital apertures into 

 the suprabranchial chambers of the inner gills, from there to the cloaca, 

 and then hack through the suprabranchial chambers to the gills. They 

 are probably fertilized during transition to the gills by sperms intro- 

 duced with the respiratory current. Some forms carry embryos in all 

 the four gill-plates. 



The segmentation is remarkably slow. The eggs of Quadrula ebena 

 observed on the morning of May 31 as in the one-celled stage, were 

 only in the two-celled stage in the afternoon. On the second day 

 segmentation had advanced to the four- and eight-cell stages, and on 

 the fourth clay sixteen-cell stages predominated. 



Specific or narrowly-restricted parasitism was ol iserved in the case of 

 Quadrula pustulosa, which is confined to Ictalurus punctatus, the channel 

 cat. A similar case is that of the nigger-head (Q. ebena), which is 

 confined to Pomolobus chrysochloris, as Surber reports. Just as the 

 louse Goniodes stylifer is confined to the turkey and Trichodectes scalaris 

 to the ox, so the glochidia of these mussels keep in their temporary 

 parasitism to one host. But there are other glochidia which infect 

 many fishes. Thus the hookless glochidia of Q. heros were found on 

 Scisenidse, Siluridaj, and Centrarchidaj, and the hooked glochidia of 

 Anodonta corpulenta were found on Scisenidae, Centrarchidse, and 

 Clupeidse. 



Some species of fish appear to be very susceptible to infection by 

 glochidia. Thus the sheepshead, Aplodinotus grunniens, is found 

 infected with the glochidia of Lampsilis Isevissima, L. gracilis, L. alata, 

 Plagiola donaciformis, and this may be associated with the fact that it is 

 a mussel-eating fish. 



The immunity of some fishes may be in part mechanical, but there 

 is an absence of histological response in some cases, and also a failure 

 on the part of the glochidia to fasten. The normal reaction of the 

 glochidia is to chemical stimulation from the ions of protoplasmic salts 

 diffused from the fluids of the fishes' gills or body. A curious fact is 

 recorded that the immersion of fish in 10 p.c. salt solution increased 

 the rapidity of infection. A fish has usually but a small number of 

 glochidia, but the sheepshead and the "herring" {Pomolobus chryso- 

 chloris) have been found with thousands. 



In general, the hooked glochidia occur on the fins and the hookless 

 on the gills, but there are some apparently constant exceptions. The 

 types of cyst differ considerably. Periods of parasitism vary from 

 twelve days to six months, and there are considerable differences in the 

 degree of development of the larva at the end of the parasitic period. 



Howard found a thread gland and larval thread in the mature 

 glochidium of Quadrula heros and Q.plicata, and he refers to the view 

 of Lefevre and Curtis that the gland is primarily an excretory organ 

 ami the thread an excretory product. They do not agree with Schier- 

 holz in regarding the thread as important in aiding attachment to 

 fishes. The hookless glochidium of Quadrula heros shows the gland 

 ami thread more highly developed apparently than in Anodonta, and it 

 has the habit of external or fin infection. This suggests that the thread 

 has some function other than excretion alone. If it were excretory 



