SIGNIFICANCE OF LARVAL MANTLE OF FRESH-WATER MUSSELS. 
205 
not covered by the mantle cells. The groups of sensory cells, although of ectodermal 
derivation, consisting as they do of modified mantle cells, nevertheless atrophy 
soon after encystment takes place and have no further interest. 
Thus, the mantle cells constitute the greater part of the cellular structure of the 
glochidium and, with the exception of the adductor muscle and the small group of 
cells of the endoderm, foot fold, and lateral mesoderm, they represent the main 
organogeny of the glochidium, so that the first encystment stage might well be 
called the mantle cell stage, since the mantle is the 
dominating structure during this period. 
A characteristic feature of the large mantle cells, as 
has already been mentioned, is their heavy granulation. 
These granules, which give a firm, dense texture to the 
cells, are so large as to be conspicuous elements, and in 
their staining reactions they have the same relations as 
have the zymogen granules of the liver cells of the more 
developed larva. They are thus probably cells with a 
secretory function and may be capable of forming a diges- 
tive ferment that would make the larval mantle of the 
glochidium a digestive organ, constituting an anomalous 
condition in view of its ectodermal nature. 
This granular feature is readily seen through the thin 
shell of the glochidium when viewed alive, and it is 
especially noticeable with such forms as Anodonta, where 
the mantle cells have a brownish tinge, which gives a dark 
brown coloration to the glochidium. This granulation is 
a temporary character, however, of the glochidium only, 
since it disappears soon after encystment takes place, as 
will be explained later. 
PARASITISM OF SPECIES OF ANODONTA. 
Since the hooked glochidia are fin parasites and the hookless gill forms, the 
conditions underlying their parasitism are not of the same fundamental nature. 
Faussek, Harms, and others have found certain anatomical and functional char- 
acteristics of the developing glochidia typical of European Anodontas. Herbers 
(1913), corroborating previous work on Anodonta, says that its developmental 
and anatomical distinctions are none the less applicable to certain Unio and Margari- 
tana glochidia. Although the first accounts of parasitical conditions were very 
general, yet the subsequent studies have been all that could be desired, and at 
present we have a knowledge of the details of encystment stages of development 
that allows a full comparison in additional studies. 
Since previous work has been confined chiefly to the hooked glochidium and 
since most of the deductions have been drawn from the studies on the develop- 
ment of these forms, a discussion of our present knowledge is in reality a discussion 
of the hooked type of glochidium and will be referred to as such. 
Fig. 2. — Section through glochidium 
of luteola, posterior to adductor, 
showing area not covered hy large 
mantle cells. I . m. c., large mantle 
cells; l. j>., lateral pits; end., endo- 
derm; mes., mesoderm; in. m., 
inner mantle. 
