SIGNIFICANCE OF LAEVAL MANTLE OF FRESH-WATER MUSSELS. 
217 
the cells becoming vacuolated and large spaces appearing underneath them, so 
that they apparently were thrown off in a mass. During encystment this might 
probably be replaced by resorption by the remainder of the larval tissues. 
OCCURRENCE OF A PLACENTAL-LIKE RELATIONSHIP BETWEEN LAMPSILIS 
LUTEOLA AND HOST. 
In Figures 7 and 8 there is pictured a very peculiar condition observed with 
encystments of Lampsilis luteola at nearly the close of the parasitic period. This 
illustrates very well the presence of a fold of host tissue within the valves, with the 
larval mantle cells grouped together near the tips of the valves, greatly extended 
and in contact with the host tissue. The involution also shows very well the red 
blood cells within its interior. With this doubtful arrangement there is a possible 
direct passage of the lymph from the involution to the mantle cells, which would 
establish a placental connection between mussel and host. To what extent the 
retention of the fish fold occurs has not been determined at the present time. In 
one lot of encysted mussels involutions were determined in 47 cases out of a possible 
51,4 being met with in which its occurrence was doubtful. 
This condition has no precedent in previous conceptions, since it has been 
conceded that all inclosed tissue lying within the valves undergoes a complete 
disintegration. A placental-like relationship is made possible, however, in this 
case, by the nature of the larval mantle cells, which exhibit a pseudopodial capacity 
at this late stage. Reverting back to a comparison between the hooked and hookless 
glochidia it is seen that the structural differences modify the character of the 
attachment of the host, so that fin infections vary to some extent from gill infections. 
With the former the large hooks bring about an extensive laceration of the entire 
tissue within the glochidial valves, and the “bite” is usually hard enough to almost 
sever the involution from the rest of the tissue. Very little connective tissue as a 
rule is inclosed on account of the extreme thickness of the epidermis, and after the 
first two days of encystment all that remain are a few strands of connective tissue or 
cartilaginous or bony material. 
The hookless form of glochidium as represented by Lampsilis luteola makes a 
clean perforation; there is no tearing of the tissue or any laceration, as is character- 
istic of the hooked form. As far as the author could determine the initial bite cuts 
down only through the layer of epidermal cells and does not extend through the 
connective- tissue dermis, although this is very much constricted at the junction of 
the valves. One can readily determine the relation of this connective tissue to the 
remainder of the gill filaments. This represents a noticeable variation exhibited 
by the hookless glochidium and undoubtedly is the result of more specialization, 
dealing, as we are, with a more degenerate form. 
The disintegration and ingestion of inclosed tissue is concerned largely with 
the cells of the epidermis, the connective tissue not having been perforated, giving 
rise to the tendency that has already been mentioned for this portion of the host 
tissue to persist within the valves. Figure 6 shows this condition very clearly at 
the end of the third day of parasitism. No cellular structure can be made out, so 
that it consists of connective tissue alone, with the epidermis completely removed. 
